On an NA car, it's proven time and time again that with an exhaust system you need to extensively flow test it to make sure that it retains back pressure, this is a FACT and a very good reason to buy off the shelf, this is extremely important for the cat-back system but even more so when it comes to downpipes & manifolds. For a turbocharged cars, it rarely matters who provides you with a cat-back system as the turbo charger and the design of the downpipe produce the back pressure but you've admitted youselves that the 'chains' do not always provide the best workmanship, sadly 90% of market is controlled by these said chains and you'll do well to find a handful of independant reputable fabricators in any region, I for one have none local but 4 Powerflow centres within a 10mile radius.
It's up to you, take the risk or buy with confidence from someone who know's what they're doing. Lucky you if you happen to have John Smith's Auto's est. 1950 right next door who knows his head from his @rse but for most of us we have Dave, the school drop out with wonky eyes and missing teeth who could.not.give.a.f*ck.
Do me a favour, firstly don't be so f*cking rude and secondly next time you disagree with a point that someone makes, be grown up about it and put your opinion across like a respectful adult. This may
help:
"I see your point JamJay but from previous experience I disagree and this is why...".