Exhaust back pressure is so misunderstood and a bit of a myth, wether its petrol/diesel or turbo/na, its all about getting a continous stream of exhaust gasses at the correct velocity and timing, if a pipe is too wide then exhaust gasses cool and expand before they have left the system and cause restriction, if its too narrow then the exhaust itself is a restriction.
You need pressure in the exhaust inorder to maintain velocity and allow the gasses to be pushed out in an efficient manner, in theory a turbo vehicle can run without an exhaust as the turbo itself is a restriction and the headers will be designed to keep an efficient flow within that part of the system, as we cant have no exhaust in the UK you still need to fit one so it has to be of the correct bore, no silencer is needed although it may be too noisy and for that reason they are fitted, they are not part of the tuning though. For a N/A car then the design of the exhaust is paramount, it needs to be made so that as each cylinders blow cycle isn't met by the previous cylinders blow cycle, if this happens then the engine has to work harder and power will be down, this is where tuned headers come in as these keep eaach cylinder seperate untill further down the exhaust, these still need to be tuned correctly though for good power. The gas need to get out the exhaust quickly and whilst hot, a big bore will stop this happening hence people bang on about large bores stopping back pressure, it actually increases it instead of allowing too much flow, this is where people go wrong...
Anyway, with a turbo system keep to the original bore but make the exhaust as short as possible and all will be ok...
