mafs are not the same according to my tuner.
Doing a side-by-side comparison of the MAF scaling, the curve matches perfectly. The only difference is that the FXT scaling is mapped out to 4.69, where as the STi scaling is out to 4.92.
This tells me that the MAF sensor is the same, so is the diameter of the tube in the airbox. I really doubt there is anything different in the rest of the airbox.
So, unless you are consuming more than 300g/sec, moving to the STi ECU doesn't make any sense. Considering it would take me 5 seconds to copy the STi scaling over to the FXT ECU, it really doesn't make any sense.
To the OP: You will not find any gains on a stock FXT by moving over to a STi ECU. If you want more boost, get tuned. If you insist on using the STi ECU, go for it. It won't hurt anything. But you will actually
reduce the amount of boost you are running. While the target boost is much higher on the STi ECU, the accompanying initial and max WGDC's are lower. You won't hit the targets....