Hi

I was surfing the internet for a proper performance exhaust system for my 2008 Volvo s40 T5 and would like some advice please. Seems that mostly a 76mm (3") downpipe is used and then the rest of the system is reduced to 63mm and the cat is also removed and replaced by a catfooler. I have however came across 2 chaps that said they have experienced boostcontrol problems after installing the 76mm downpipe. Apparently the size of the downpipe helps to control the amount of boost ? I was also advised to do the downpipe and catfooler but to not replace the backbox to keep the backpreassure correct by some others ?
My goal is to get more power and would like some of the forums expertise please.