Just wanted to report back that I have added thermal pads to the bottom heatsinks on the weekend. While everything went well, the expected cooling benefit hasn't really been met in my opinion.

Yes these heatsinks do get warmer now, so there is better heat dissipation. But not significant in a way to have a measureable impact on watts or hashrate. I had hoped these pads to be a thermal bonus for my efficiency setup (550Mhz, undervolted), but if at all they are probably more of use for the heavy overclocking case.