Well good news everyone!
I talked again with the electrician, this time showing him the mosfets of 290/290X
He said they keep their ideal performance even @100c+ and he also said they are high quality, and very good generally - he says that very rare, he wasn't so impressed by the inductors he rated them as average, and recommended to not exceed 60-70c on them for good performance. He works - repair, expensive gear, so for him to say for something that is high quality means alot
So ye, anyone who plant to buy some non-reference cards, think really hard before you do it. He confirmed to me that, if some of this cards ain't oc well, there is no - or very very little chance this card VRM design is holding you back
For now, no card beats reference ones regarding vrm performance, maybe Lightning/Mars will do it, but not sure, the mosfets they are using are top notch, he was really surprised to see such linear performance even at very high temps - he didn't expected that at consumer grade products, iR - vrm's on this cards rocks!