haven't seen any recent mag articles on the subject but i know of someone that knows firsthand... lordduckhunter will tell you that moriwaki or sato pipes will make the most hp (however marginally) out of any slip on you can find. granted those aren't the cheapest but if you want the extra 2hp there you go. i personally am gonna go with yoshimura. they have good build quality, performance, and sound all for a reasonable price. check discountmotorcycleexhaust.com and see what you think.
I ran an Erion Titanium slip-on on my 02 F4i. Very nice exhaust. Now running an Akrapovic on my 954. Personally, I do like the Akrapovic a bit better, but being on a different bike, I can't really make any other comparison.