[Pi Case and Raspberry Pi 2 Broadcom die size]

I think that it's still a better conductor than the air itself :). I think that gluing (using a thermal conductive glue of course) a thin flat piece of copper on the heat sink post may be doable. Then replace thermal pad with some good thermal paste like Silver Arctic 5.

 

Oh yeah, it's better than air for sure, I was just saying that in general, thermal pads are worse than direct contact (with paste).

[Pi Case and Raspberry Pi 2 Broadcom die size]

I doubt you'll be able to measure the gap, but if you go one up, i.e. 0.5mm from whatever the supplied pad is you should be good. You'd probably do well to not put the pad in the center of the post, but place it so that it covers more of the chip.

 

The problem with a thicker thermal pad is that it loses a lot of effectiveness, and starts to become more of an insulator then a conductor of heat.

 

Thermal pads don't actually help transfer heat amazingly well, the best solution would be two perfectly flat pieces of metal touching directly, but that's not possible, so we use thermal pads/paste.