Yeah, I know what you're talking about. This R6 is my eighth or ninth bike and not a one of 'em kept their stock pipe for very long. Well, my FZ-1 did, but I ultimately swapped that out as well.
Anyway, a slip-on alone isn't going to smooth out the powerband completely. You'll have a bit more power and torque above stock at most points in the rev band, but the manner in which it's delivered is still going to be pretty much the same.
Now a slip-on combined with a properly set-up PC might go a little farther; then you can adjust fuel delivery to go with that free-flowing slip-on. Still, the engine will always be somewhat of a peaky beast by design, but you should notice a fairly appreciable gain in throttle response and drivability after adding a slip-on and a proper map.
As far as quality products go, I can recommend anything by Yoshimura (really good stuff even if they are everywhere), Scorpion (killer tone without excessive noise, no-repack, and a good price), Akrapovic (pricey but gorgeous and sounds fantastic), and then Two Brothers, which I thought was well made but still a bit on the loud side for me personally.
Those are the ones I've had first-hand experience with and can heartily recommend.
(BTW, I'm currently drooling over the Arata slip-on for my R6....oh man is it gorgeous! Big dollars though.)