I've been flying 5" props on a 6" frame for a few weeks now and it's really smooth. Been working on the tune on this quad and it's pretty much locked in. Build Spec:
Catalyst Machineworks Superlight 6"
MB Primo 2207 2450kv
Matek F405 FC
Spedix 30a 4 in 1 ESC
TBS Unify
Runcam Micro
DAL 5050C 3 blade
Betaflight 3.2 RC5