Well, since you're soliciting opinions, I'll post mine for you. 😉
As an M4 owner myself, I'd say the GP version is the worst one they make. Not only is that think ear-splitting loud, the flat painted megaphone style does nothing for the lines on the bike. They make nice exhausts, but the GP is at the bottom of the list list. I have the Street Slayer and the sound is very, VERY close to the Graves slip-on. It's loud, but has a deep tone to it and sounds much better than any GP I've heard. Like LDH, I'm a bit old-school and the standard mount Tech 1 from M4 would be comparable. As they go, I'd say Street Slayer, then Tech 1, and then any other exhaust than the GP. 🧐
As for power, you're not going to gain anything down low with any slip-on. The motor is peaky and you've got to slip it up to get any power out of it. The slip-ons are about weight reduction, sound, and looks. Minimal power gains can be made with them if properly tuned, but what you're looking for won't happen with with that either.