Andrés Giménez is widely considered the Mets' top prospect because he's a young, impressive left-handed hitter with above-average speed and plus tools at the premium position of shortstop. The only glaring hole in his game is his power, which received a 35 grade on the 20-80 scouting scale from MLB.com this season. None of his other four graded tools (hit, run, arm, field) received lower than a 55. But could the game's No. 57 overall prospect one day show the pop to round out the profile?
He might, in fact, already be on his way to doing just that.
This week's Toolshed turns its advanced-stats gaze toward isolated slugging percentage (ISO). At its base, ISO measures how many extra bases a batter collects per at-bat. In practicality, it is used to show how much power said batter has produced. In its simplest form, the stat is derived by subtracting batting average from slugging percentage. Consider a player who has gone 2-for-5 with two singles versus a player who has gone 2-for-5 with two homers. Their .400 averages say they were equally productive. Their .400 and 1.300 slugging percentages would say that wasn't the case, but their .000 and .900 ISOs would really drive home the difference in their power production. It's an extreme example, but it illustrates what ISO will be used for here.

As in past years, the Toolshed won't be looking at ISO alone but examining how it corresponds to the expectations set by power tool grades given to position players ranked among MLB.com's Top 100 Prospects, using mean and standard deviation.
For the 2018 season, the mean ISO was .172, and the standard deviation was .057. The mean power tool grade was 52.09 and standard deviation was 6.43. Those latter numbers make some sense; while average power is considered 50, this is not an average group of prospects. These are some of the most powerful in the Minors, tipping the scale above what we might expect for "average" power.
But new for the ISO column this season, Toolshed is also going to use those numbers to produce expected isolated slugging percentage (xISO), i.e. the ISO a player would be expected to have produced in 2018 given his power tool grade. It's not an exact science, given that it doesn't take into account factors like league, level or home parks. It also doesn't produce many differing numbers, because tool grades typically are round numbers. But it does a better job of measuring production with expectations than showing standard deviations alone.
By using those measures, these were the biggest overperformers among Top 100 Prospects this past season:

Biggest ISO overperformers among top 100 Prospects (min. 100 at-bats)

That brings this back to Gimenez. 
The Mets' top prospect is listed at 5-foot-11, 161 pounds -- not exactly optimal size for power -- and was coming off a 2017 season in which he collected only 17 extra-base hits (four of which were homers) and posted an .084 ISO (.265 average, .349 slugging) over 92 games at Class A Columbia. Hitting for pop was not his forte. A 35-grade power tool was apt.

But in 2018, he started to collect extra-base knocks more regularly with Class A Advanced St. Lucie and Double-A Binghamton. Only six of his 125 hits this season were homers, but five were triples and 29 were doubles. He batted .281 with a .409 slugging percentage, good for the .128 ISO listed above. Again, those may not be killer numbers, but they do represent significant improvements over 2017 -- especially considering he posted them while being moved aggressively at age 19.
Where did that power come from? Consider Gimenez's spray chart on extra-base hits this season:

Those are a lot of opposite-field doubles down the left field line. Of the 29 two-baggers Gimenez collected this season, 21 came to the left of due center. That's extreme, but not completely out of line with the Venezuela native's general approach to hitting. Gimenez sent 39.9 percent of his batted balls to the opposite field this season, compared to 39.1 percent to the pull side and 21.1 percent up the middle. That's indicative of a slap hitter more than a yanking slugger, especially considering more than half (54.4 percent) of his balls in play were hit on the ground. 

That said, Gimenez might be able to build on this type of power production. The 2018 Futures Game participant has developed a reputation for showing a quick swing, and he takes advantage by making a good amount of contact (18.3 percent strikeout rate). As he continues to grow and mature, there's the potential for him to add more strength and eventually be a double-digit home-run hitter at the upper levels. When he did show over-the-fence power, it came to the pull side; five of his six homers this season went out to right.
Regardless, as things stood in 2018, Gimenez proved to be closer to a 45-grade power hitter than 35-grade. That won't jump off the scouting report, of course, but consider the difference. For instance, a 35-grade power hitter (according to ISO) was White Sox infielder Nick Madrigal, who had only seven of his 47 hits go for extra bases this season and all of those were doubles. A 45-grade power hitter was Nationals shortstop Luis Garcia, who hit seven homers and posted a .406 slugging percentage in 127 games. It's not killer power, but it's not zero power either.
If Gimenez can keep that up in his early 20s, he might be an even more toolsy prospect than previously imagined, and his quick climb could continue in 2019.
Among the other overperformers from the table above is fellow Mets prospect Peter Alonso, who ended up tied for the Minor League lead with 36 homers between Binghamton and Triple-A Las Vegas. The 2016 second-rounder wasn't able to show incredible power in 2017 -- partially because of a broken hand -- but he showed more than plus ability in the category this season. His 415-foot shot at the Futures Game in Washington D.C. was one of the highlights of the year. Joey Bart and Danny Jansen showed that their value comes as much at the plate as it does behind it, while Julio Pablo Martinez collected 25 extra-base hits in 67 games despite playing exclusively in short-season circuits during his first Minor League season.
Below are the biggest underperformers per xISO and ISO and top overall ISO performers from the 2018 season. Some quick words on one of the notable names contained herein: No one doubts Eloy Jiménez's pop, and no one doubts that he showed impressive power in 2018. Indeed, he finished eighth among the 55 ranked position player prospects with a .240 ISO, thanks to 22 homers and 53 total extra-base hits in 108 games between Double-A Birmingham and Triple-A Charlotte. However, MLB.com gave the 21-year-old, right-handed slugger a 70 grade for his power -- the highest power tool given out to a current Top 100 Prospect. Only Vladimir Guerrero Jr. (65) comes close. But with a metric that measures expectations against performance, lofty expectations can be detrimental. By all other indications, Jimenez should be one of the Majors' most prolific sluggers when he arrives on Chicago's South Side early next season.

Highest Isos for top-100 Prospects (Min. 100 at-bats)

Biggest ISO underperformers among top-100 Prospects (min. 100 at-bats)