Brett,
The classic approach for use of diffusion is to have the return be beyond the haas effect... the 20-22ms you refer to. We have done smaller rooms where this wasn't possible, and in my opinion this is one of the best applications for 2D diffusors such as the skyline/omnifussor types. These units spread the diffuse return in a hemisphere and as such, there is less return to the mix position (as compared to a 1D diffusor like a QRD). I believe that diffusion in the front of rooms has to be very carefully implemented. You don't want to have "early returns" even from diffusors as this will cloud imaging and the overall depth on the source material. In a 13' deep room, depending on your listening position distance from the front, it should be possible to get near a 20msec 'round trip' from the speakers (past your listening position) - to the diffusors - and back the listening position. It's the difference in the time of direct vs: diffuse return that you are concerned about.
With regards to carpet around the perimeter, is the ceiling is trapped, then we typically use hard flooring throughout the room.. if the ceiling isn't fully trapped (absorptive) then I consider some carpeting around the perimeter.