Sorry mate but the exhaust can go wrong and go wrong big time if someone builds one who doesn't know what they are doing.

The system has to resonate correctly or it will decrease flow even with a large diameter pipe. It's like if your exhaust is blowing or you drive it with only the manifold on, it will be much more free flowing but you will also notice the lack of power. This is due to back pressure and resonance frequency aiding the flow of gasses out the back which if not done correctly will reduce power.

There are replacement ECU's, that can be programmed to work with any engine. has someone tried?

How much power increase can be made with more open exhaust pipe ... and catalysator removal??