I bought a Macs tuned pipe for my OS 46 FX, and man what a difference. I went from about 12500-12800 on an APC 10x7 to almost 15000 RPMs. This engine screams now. this was th Macs pre-tuned pipe. I didn't fiddle with it, but I bet I could get even more performance if I messed with the header length and what not. But I am pretty satisfied with the power I am currently getting, so I probably won't mess with it.

This of cours is going to be a little more expensive thant the tower exhaust, but you can't beat the power.