Hmm... if you put it that way...
If your car had shite load of mods (fmic, cams, exhaust, new turbo etc. etc.) but a standard ECU with bad tune AND you just dropped in a PowerFC AND you got say a full dyno tune you could feasibly get 100rwkW extra...
It's not a common scenario but is it?
Aftermarket engine management is a forgone conclusion if you want to modify the car and the original ECU is full of flaws...
T.