Well I had mine done today........a powerflow custom!
I have a huge centre exit "oval" its positively massive!
Its straight through, with just 1 box and it doesnt seem to be down on power that much.......in fact it seems to really hit the rev limiter much easier.
i had a cherry bomb on my sei. Its nosiy but didnt feel any different... Im gonna get a quote from powerflow, or theres this place in cardiff that do the same, Line something i think