Just to add to Demon's comment..
SLOW (SHO) SHOP sells a 63mm t.body and shows a dyno with an entire 2 HP gain over a non optimized 60mm stock SVT part on a modded 185 HP SVT. Just modding the stock part has been shown I think by Brad Noon to add 2 HP or so.. This confirms that on even moderately modded 2.5Ls, there is little to gain with a bigger t.body over just modding the stock SVT part.
