I would agree that Muzzys does extensive R&D on the Kawi's. I think that for the street, qualilty in construction is also important. I've found the Yoshimura systems to be constructed better (more solid). After owning both of these, and now a full Micron system for my ZX9R, I've found the Micron to be even better than the Yoshimura.
I also find the Yoshimura to have a deeper sound than the whiney sound of the Muzzy. I like the deeper tones better, and relatively less attention getting, if you know what I mean.
As far as slip-on vs full system, if you're looking for more performance, it's obvious you'll want to go full system. The pipes are bent and tested for optimum performance, vs, a slip on isjust designed to breathe better with what you've got (stock header).