I have heard/read many comments on the Apexi unit with stock twins. Most all of them were very happy with the results from adding the Apexi FMIC.
Just about all of the comments I have heard about running aftermarket FMICs with the stock twins have been positive. Most claim that the car is better able to hold peak rwhp to redline. Usually, the stock twins peak and then hp drops pretty dramatically towards redline.
Most common complaints with the Apexi FMIC are that it is smaller than the Greddy FMICs and the larger HKS units. Also, it doesn't come with the upper hard pipes.
I'll bet that you will be pretty happy with the performance gains you will get with the FMIC.
I've got the apex and don't have any real complaints. It was a pain in the ass to get the driver side piping to reach up to the TB (might have been because of the hardpipe from the blitz BOV kit) but all in all the install wasn't too bad.
It would've been nice if it came with the upper pipes. The core also sits pretty low.
but as someone else mentioned, it sits really low, so low in fact that I cracked one of the mounting tabs. I hit a parking pylon at very very low speed, and nonetheless, it cracked the fmic, had to take it out and get a shop to weld the crack