The R6 has a MASSIVE "mid muffler", that 21LB square box you can see sitting right behind his muffler. Even with NO muffler an R6 isn't loud when you've still got that mid muffler
If you really want to make that M4 sing:
https://www.dan-moto.com/DM_INT/ind...ucts_id=1067&zenid=885924piervqgjvahvol2d28k0
If you don't have a power commander to correct your fuel/AFR for both the slipon plus having no more EXUP valve you can get your ECU flashed with a proper fuel map and that'll help bring back some of the power down low, that you've now lost.
If you're buying the midpipe also look at getting smog block off plates (unless you like lots of pop, backfire and flames on deceleration).