You can get a big gain from their pipes but there are a lot of ifs involved. First, if you have an 875 engine with stock cams, you will gain little or nothing because of the crappy valve timing. Second, if you have already opened up the intake and didn't realize any significant results because of the restrictive stock pipes, you will gain a lot with better pipes.
A couple of years ago, Thunderbike published a lot of their dyno tests on this site and their thoroughness in testing and results were impressive.
