not so sure about the no back pressure thing Very Cherry tbh
maybe i should put it into context a bit more, the backpressure isnt really influenced when you talking about a modern automotive exhaust as such, as any pressure wont particularly come from the bore of the tube but from any resistence such as bends or obstructions. so not really a concern from a modern day car, in the case of the golf this will only really happen with over complex exhausts that have to move through the length of the car utilise series of complex bends for clearance over undercarraige 'furniture'. the standard route is normally the most direct to avoid these complications.
this means that as long as the shape of the exhaust and route hasnt changed dramatically and no dramatic additions are made to it from the factory design, then the bore when talking in 0.5"s in difference wont come into play
theres more risk of back pressure from running to small a bore rather than too big as the engine cylinders fire at different times, which mean exhaust leaves them at different times thus creating waves of gas. the wave from one cylinder may not completely vacate if the pipe is too small before the next pressure wave makes its way into the mix, in the same way that overcomplicated bends and obstructions can create back pressure so does this and a restriction in the engine's exhaust system can restrict the engine's true performance possibilities
happy mediums, but depends on how much modding you have planned as to how serious you make it, if its just for a bit of noise and hopefully a few horses then its nothing to worry about in that sense
