I've ridden a few different Cinches and Mutants. I hated them all. If you want speed Flow is better. Do you get strap relief? No. You don't need it. That big strap distributes strap pressure better than trad straps so you don't crank the strap like you do for trad. It will feel different yeah. But you will get used to it and then you can enjoy quickened edge to edge performance. That cable triangle thing works. Cinches are heavy and they don't adjust right in the toe area. They force you to have too little toe overhand regardless of boot size/binding size and you lose leverage. Gnu's felt like 4 year old Rides. So if you really liked the pink Beta Mvmnts you'll be able to get all nostalgic with the Gnus. Except have to deal with less adjustability.
If you're in it for speed, Flows are superior. If you're not and you don't want Flows, then there is no reason for rear entry.
Go ahead, drink that kool-aid.
FS: Forum Sauce 142 $200 plus shipping
FS: Flux SE30 $75 plus shipping