In 3 consecutive runs of Unigine Valley, these were my max temps: (GPU is overclocked to 1200/1680)
GPU: 55 C
VRM2: 58 C
And it was a bit fiddly to get it in, but it's very simple to do, make sure you have sinks on VRM1 as it gets quite hot and that u check the clearance from the bracket and there isn't room for massive heatsinks, those temps on VRM2 were without Heatsinks.
Ahahah this is not true.
I have prolimatech mk-26 with 2 pieces of the reference plate on vrms ( and Fujipoly Sarcon 17W/mK). 2*140mm fans on the prolimatech, 2*140mm fans on the side.
I'll post photos but i can say that, with stock voltage i achieve 55C on vrm2, 68C on vrm1.
You cannot get 78C with little heatsinks on vrm1 ( or with not heatsinks at all). The reason why i don't choose liquid aio cooler for r9 290 is bad dissipation of Vrm1.
A believable result with, i imagine, +150mV ( at least for 1200/1680), and Nzxt g10 is 95C on Vrm.