I'm having this exact same problem. I switched from an EK supermacy evo to the EK rampage VI extreme monoblock, and while I had the thing apart, I delided and applied conductonaut liquid metal. I put everything back in, and temps went up at idle by 5c, and on cinebench they 70+, and AIDA64 cpu stress test pegs them as high as mid 80s. It was never this high before.
I took everything apart and redid everything, reapplied LM and new cryonaut inbetween the monoblock and the IHS, and now idle temps are in mid/high 40s and stress tests hit almost 90c. When I was looking at the monoblock, I saw that the part that contacts the IHS is a small circle, and the supremacy evo was a flat square, and I was wondering if there was just poor contact and parts of the IHS that weren't even touching the monoblock. Also, I was wondering if it could have been when I used the little tool included with the Der8auer delid mate X that clamps down the IHS to the die pcb, maybe that bent it in? I'm at a loss for words on this one.
I'm going to pry things apart again and reinstall my supremacy evo and see if the temps go down. If they do, then there's something wrong with this block. It sucks, because when it's set up, it freakin awesome. It looks better than any liquid block I've ever seen. Shame if it's bad.