most stock headers on any newer spoirtbike flow so well that unless you plan on doing internal motor work, then a slip on or half system is all you will ever need.
some bikes, like the 929/954 and the new gixxers, with the valve in the exhaust, actually LOSE power in the lower and midrange with a full system. they make a bit more up top, but 90% of the riding on the street isn't done at or near redline.

i haven't heard the gytr pipes, so i couldn't tell you what they sound like, but they look awesome. i saw a set on a yellow '06 r1 the other day. the graves are moderately loud, with a deeper tone than some of the others that i've heard.