go to RPI directly...eloy is a good guy to deal with, no BS and knows his sh!t..
in terms of loundess, that's so subjective it's hard to answer, their shop m3 has a full gtm set up IIRC, so perhaps E will let you hear it..or someone local can meet up with you to hear it. If it is loud you could always consider their GTC set up which has been said to me more drone and volume friendly.
Craftmanship and gains I would say is very good, welds and bends are
RPI exhausts are fully bolt on to the stock system, I've read of some install problems all seem to have been from install error rather than build quality
In terms of PC'ing the cans, again that's subjective as some do not like the SS can look...I would go for PC'd cans personally..
good luck with your decision and congrats on your new to you M...