If your cats are plugged, then the backpressure and resulting heat will build up around the exhaust manifolds/pre-cats. Like kbannister and alabbasi say, check for heat shielding and double check the floor vents when driving just to make sure no hot air is creeping through.

When you say excessive floor heat do you mean you feel hot air or is it the temperature? Pull the carpets behind accelerator pedal and see if the floors are hot to the touch.