How much insulation do I really need?
Since 50% of the bed is uninsulated, you're definately into diminishing returns as soon as you start adding any insulation.
With that area, I think you are looking at 1.2W per kelvin for a 2mm thickness. 

I'm guessing a bit with these powers, but roughly, maybe from 100 W un-insulated, 75 with 2mm, 60W with 4mm. You can get a reasonably accurate measure of the power by looking at the duty cycle of the heating element.

Actually, its not clear if your primary goal is to reduce energy/maintain a very high temperature, or speed the initial heating. You can place a temporary sheet of cork on top of the bed (preferably extending over the edges to prevent convection) and this will significantly improve heat-up times.