775s motors are powerful and on the correct ratio work great and maybe people still use them, IIRC Explosion uses 775 and Shrapnel the vert runs 550s (and voltage DO NOT OVERVOLT them to 6s 22.2v)
Kaizen uses 4x 775 motors in it's drive system. From experience, I wouldn't recommend going past ~24v or so on them. I run them on 8S LiFe batteries (26.4v nominal) and, unsurprisingly, they'll cook under high load for long periods of time. I would agree that 550s are probably not going to work beyond 14-16v or so. I seem to recall a post on here at one stage about how much you can overvolt a motor, might be worth finding.