@Bojamijams: there is no flow above the VRMs because of the thin copper block. If we want to have flow there the block must be much higher (= more expensive) or we had to use a bridge like for one of the HD48X0 blocks (see example, also more expensive).
The VRMs can handle very high temperatures (which you will never come close to) without any problems so I see no reason to worry here.

@Utnorris: there is no coating.