When people think of Mike Leake, the word "upside" does not usually come to mind. Leake has been a consistently league average pitcher throughout his career, posting between 1.4 and 2.3 fWAR in each of the last five seasons. He does not post great strikeout rates or possess elite stuff. In many ways, he is the prototypical high floor, low ceiling starting pitcher, and any team that acquires him should know exactly what they are getting.
If there is one area of upside for Leake, it lies in his ability to outperform his peripherals. In his career, Leake has posted a 3.88 ERA and a 4.21 FIP in 1083 2/3 innings. His ERA-FIP gap is especially pronounced on the road, where he has posted a 3.48 ERA and a 4.16 FIP. While Leake will no longer be pitching his home games in Great American Ballpark, it is difficult to say whether he will be able to continue beating his peripherals in this manner. ERA and FIP do not regress Leake's above average HR/FB rate to league average like xFIP does, so it would not be accurate to say that Leake's ERA-FIP gap has anything to do with allowing more home runs at Great American Ballpark than on the road.
In fact, Leake has posted a lower BABIP and a higher strand rate on the road in his career, and these factors appear to be driving the large gap between his ERA and FIP. I checked to see whether pitchers tend to post a higher BABIP and lower strand rate at Great American Ballpark compared to other ballparks, and based on what I found, this does not appear to be the case. Since Leake first broke into the league back in 2010, Reds pitchers have allowed nearly identical BABIPs at home and on the road (.284 at home vs. .285 on the road), and they have actually posted a much higher strand rate at home than on the road (76.2 percent at home vs. 73.5 percent on the road). In other words, there does not appear to be anything inherent in Great American Ballpark that would suggest that Leake's career ERA-FIP gap on the road should be a new normal for him now that he is with the Cardinals.
With that being said, it may still be fair to expect Leake to outperform his FIP somewhat. He fields his position well (27 career defensive runs saved) and generates a high ground ball rate (50.2 percent in his career). Both of these factors appear to help pitchers outperform their peripherals.
It should also be noted, though, that Leake has pitched in front of one of the best defenses in baseball during his career. Since 2010, the Reds as a team lead all of baseball in FanGraphs' defensive runs above average metric (Def), and they are top three in UZR and DRS. Over this period of time, Reds pitchers as a whole have outpeformed their FIP by .16 (3.84 ERA vs. 4.00 FIP). The Reds' defense has given a boost to their entire pitching staff over the last few years, and we should not discount the role that team defense has played in Mike Leake's ability to outperform his peripherals.
If nothing else, Leake will at least be boosted by the pitcher-friendly nature of Busch Stadium. He has a career HR/FB rate of 13.7 percent , which puts him 9th among qualified starters over the last six years. This number will almost certainly come down, although it is unclear how much, since Leake still owns an above average home run rate away from Great American Ballpark as well. His career xFIP is 3.81, so this gives us a decent idea of how Leake would be expected to perform with a league average HR/FB rate.
For what it's worth, Steamer does not seem to believe in Leake's ability to significantly outperform his peripherals. It projects him for a nearly identical 3.98 ERA and 4.01 FIP in 201 innings. I tend to agree with this assessment, as I think that most of Leake's ERA-FIP gap can be attributed to the Reds' defense. As a ground ball pitcher with low strikeout numbers, Leake may have benefited even more from the Reds' stellar defense than other Reds pitchers.
We cannot dismiss the possibility of Leake outperforming his peripherals over a short period of time, and he will likely benefit somewhat from pitching his home games in Busch Stadium, but it is probably unreasonable to expect Leake to continue to outperform his peripherals over the life of his contract.