The JB4 is blind to whatever tune you're using, it only sees data gathered from the sensors it intercepts and what it reads from the OBD2 cable. It'll work the same on a tuned car as on a stock car, but you couldn't run any of the pre-programmed maps, as they would try to run way too much boost on an already tuned car. You would need a custom map 6. If your tune is already appropriate for the fuel/hardware you're using, then there won't really be any benefit to running a JB4 on top, but it can be useful for stuff like adding boost when you put a few gallons of e85 in the tank, or running race fuel temporarily, or if you're running water/meth injection, etc... I used it to compensate for when I did my IS20 swap back before there were any IS20 tunes, and ran a 13.3 @ 110mph with it, compared to 14.6 @ 96 with the IS12 on the same flash (APR Stage 1 91). In the OP's case, I'm not sure a JB4 will be worth it unless they have an e85 station nearby or plan on adding other hardware that they can't easily tune for.