[Fixittt]

I would stick with the 5mm thickness.

This thickness will help spread the heat thrueout the block, and not have a concentrated center hotspot. I had a block with a very thin base. 2 to 3 mm. and got worse temps. My best guess is that I saturated the material with heat faster then it could move it. so in turn the core got hotter. But with an identical block with 5mm base I seen a very good sized temp decrease. My guess is that the heat was able to be pulled away buy the rest of the blocks material. but when I moved up to 7 mm i noticed that overall system temps ran higher.

just my observations.