Hello everyone I can not believe it, but I just solved my problem.
The low hashrate reason was in deed the BIOS being wrong. It caused the GPU to have a wrong power drawing from its 8-pin port.
I wrote to Palit/Gainward tech support and they gave me the correct BIOS.
Now the GPU gets 41.5 MH/s with 125W as it should!!!
I just want to say big thanks to everyone who helped me here. I couldn't do it without you guys!
I will leave this thread with [SOLVED] tag. I think it can be useful in case anyone in the future has the same or similar problem.