Hey Gordon,
From my experience with heat dissipation, I don't think you're going to get much better dissipation by increasing the dimensions of the aluminum plate. Heat from devices like this transfer from conductive to convective in a fairly small locality. You may get better reflectivity, though.
If better reflectivity is a goal, polishing the plates may bring good results.
This is a larger heatsink polished for better thermal conductivity, it was accomplished with ever decreasing grades of sand paper and polishing compounds:
