Here is a graph showing GTSS vs -5. This was done on the same car and on the same dyno.
http://4.bp.blogspot.com/_vhpEGyEVhes/SDya...00-h/GDZIL4.jpg
the -5 were 500 rpm behind the GTSS up until ~4600rpm where they draw level but at 5500rpm the -5 pull ahead of the GTSS.
At the end of the day, make your own choice. Here is a couple of good threads for you to check out. Take note of the guys that had GTSS set ups and moved to -5 set ups, they say they prefer the -5s. It depends on what you are after.
I bought a set of -5s
http://www.gtr.co.uk/forum/99765-gt-ss-gt2530.html
http://www.gtr.co.uk/forum/96210-5s-7s.html