In my experience, the stuff from Japan fits with OEM perfection - or at least this is true with HKS and Apexi. The cheaper stuff I've dealt with had issues with alignment (rubbing on chassis or bumper skin) or mounting (misaligned hangers or flanges in the worst case). One generic exhaust we ran had issues with ALL of the above.
Not saying all cheap stuff is bad. We ran a Che header with no issues here.
+1 to japanese parts... if you dont mind paying for the ultimate in quality (eg. $600-1200 for a cat back exhaust for our cars depending on brand and configuration) then you can get something that will be OEM quality or better, that will fit perfectly and probably last a lifetime.
I have only ever had good experiences with buying original brand japanese parts, perfect fit and function every time. Yes you are paying for a brand, but you are also paying for quality in the welds and attention to detail etc.
Its worth looking into who can source you some of the genuine japanese parts, some places are crazy over priced while others can get better deals so its worth shopping around and not all JDM brands are as expensive as others.
Yes a genuine amuse titanium exhaust will set you back $2000 for a cat back but other brands have more reasonable prices, for example you can get a kakimoto exhaust for around $600. People get blinded by the over priced stuff and forget there are more reasonably priced items coming out of japan still. If you want to save some money, just wait till the exchange rate is in your favor.
A real enthusiast will appreciate the difference in quality between going out and buying an exhaust from Kakimoto/HKS/GP Sports etc. or an ebay china item.
By now you can probably guess im a bit biased towards genuine jap parts, but from my experiences they have been the only parts that have never disappointed me. Everything else there has always been some slight issue that has annoyed me and made me wish i spent the bit extra for something genuine.
In saying that i have che extractors but it is only a temporary solution as i did not want to gut the stock cats and will likely go turbo/engine swap in the future so the TRD headers i wanted would not have been a good investment in this case. They seem to work fine, but the mounts had to be modified to fit and the heat shield also had to be cut for it to fit as well.