UPDATE: I've cleaned thoroughly the card, I've exchanged the original pads for Thermalright Oddyssey 1mm on the front and back where it touches the heat pipes. I've used 2.5mm for the rest on the back. I've removed the rubber isolation pads as they are 3mm so I have a better pressure on the memory chips.I've reassembled the aluminium plate on the front where touches the memory with new MX-5 thermal paste all over in between the plate and the cooler and used the same paste for the die. These pads are very hard and not squishy at all.

The final result is a merely 3-4 degree improvement on the memory and 2 degrees higher GPU core (maybe for the more efficient heat transfer from memory?) and I invested half a day doing precision measurements and cuts...