I bought a single fatboy pipe by Fast ($100) for my summit 670. Installed it for the first time yesterday, took some time to dial it in, it sounds great but I can't say it was a very noticeable difference performance wise. I just ordered a MBRP can for it so if anything I'll be able to shed some weight.
Maybe you'll have better luck with the twin pipes.
Never used them. I ran Jaws pipes on my 670s and they worked great. Change the pin weight, and used a 50/47 helix with spring on c6 ( I think) in the mountains and always worked great