I think what Korg did to STS in the PA4x can probably replace how i use the T5 registration buttons, its a quite advanced system.

i have both the wheels of T5 and the joystick and ribbon of the PA4x (on my Kronos) they just work different, but the PA4x are freely assignable, its just a matter of getting used to.. but in this case i think the PA4x has more to offer. In the end, the best combination is having both, the wheels on T5 and another board with the joystick and ribbon

And about the mute buttons, i think thats where Korgs touchscreen inteface comes in handy, the style track mute buttons are available trough the touchscreen

Personally i would love to see the ACMP button on T5 being replaced by 3 buttons.. DRUMS BASS and ACMP... allowing for real time swithcong on and off Drums bass and the rest of the ACMP channels. touching a single time toggles the swithc, touching two times in quick concession switches the others off and makes it solo.. Also the bass inversion and hold tempo buttons on Korg are very usefull

This is whay i prefer to do when playing...