Are you referring to front or rear brakes?
As for myself, I use Hegar4's up front, but Hegar makes 5" rotor brake system as well, just don't know about wheel clearance. MCP makes an Enduro front brake, with 6" rotors. For rear, I use 10" rotor and MCP cast caliper. That seems to be all I need, HOWEVER the Righetti Ridolphi kart brakes are kinda sexy, vaned/vented rotors, dual caliper and the like, but those are more for a 120+ MPH metric shifter kart.
If you have a heavy chassis I can understand you wanting more brakes. BUT if its a hard pedal with no stopping power issue, something's not in balance.