so far im yet to see a direct comparison before and after with after market inlet manis on these cars. my mate has a race tech one on his and it was quite a ball ache to fit it, a good few things had to be adjusted and modified. the tps caused him alot of bother.
You can chop and slightly modify a standard Mani where needed tho, bigger plenum chamber will give a increase on the dyno! I know this because my old man custom makes them for cosworths.