[MpG]

I wouldn't be surprised if this hampers the overclockability of the cards. Less space between signal traces increases the chances for cross-interference at higher frequencies. But that's only assuming something else in the cards isn't a bigger limiting factor already.

Another thing - I couldn't say for sure, but a thinner PCB is obviously not going to be as physically rigid. A heavy full-cover waterblock might bend them a little more too.