If you mod the Bios they are out of warranty.On the other hand you get 25-26 Mh at standard and 29-30 Mh at modded Bios.It is a preferance.
Indeed I do not recommend you to mod the bios because its dangerous and you will void your warranty and it can due to that provide you some big problems too.
I would not do this.
No one will discover that the bios has been modded if u restore it before sending it back
IMHO they will not even check