Well I went with Two Brothers High Mount CF one. Of course you should really go with a full exhaust not just the slipon regardless of the manufacturer. Honda put on too much emissions crap which adds a bunch of weight and I'm sure it saps power.
I love the sound of my 2 brothers pipe. It is getting louder by the day. Two Brothers reported a 13 hp difference with their full system and a K&N Filter, plus it saves close to twenty pounds.
I am also in the market for a slip on for my VFR800. I've heard that full systems take away from the mid range power of the bike and make it not as pleasurable to ride. I mostly want it for the sound and the look. I've heard that staintune are very unique and good quality. I want a high mount pipe but I don't want to lose any passenger footpegs. I also have a tailocker on my bike so the wolf system is out. I also want to keep my center stand.
The high mount two brothers pipe doesn't lose the rear pegs. So that's not an issue. The full system just fits exactly where the old system did. The pipe even has a rest for the centerstand machined in.
I think the two brothers is the best out there, along with Erion for Honda's.
I don't like the look of M4's. They look stubby and cheap IMO.
I don't think I've felt any loss in power in the mid range after I installed the system. The only thing that I would have changed is the fact that my FI light has been on for 4,000 miles. But it's running as good as it ever has. And the bike just keeps getting louder as I pile the miles on.