Had success on replacing my Early 2011 with faulty GPU with a Mid 2012.
Tested with 8+ hrs of GPUBench with 3D rendering and everything was ok. I didn't had big issues on putting LCD cable on logic board, just bent a little bit the external housing using a nipper.
Board was used from eBay, just replaced thermal paste with an Artic one, I'm happy I resolved this thing :)
Thanks for this discussion, this was my inspiration for resolving this problem, now I have a brand new MBPro :)