My computer can handle it perfectly. And I've tried that awhile ago. Still no difference at all. It plays every game I play fine with no lag. But when I have my fps maxed at 100 because I dont like to see screen tear all the time. It will drop down to 75 which you cant see a difference bewteen 100 and 75 anyway. I just think thats a huge drop.
Dual monitor will tax your gpu power..... (disable one when playing demanding games is good idea if your machine cannot keeps it up)
and u cannot RMA because it just lost a little performance and nothing actually broken..
edit: as the answear for your q, ussualy most store will handle the RMA for u if it not a very long time since u bought it. if it past the time but still under manufacturer waranty u can contact the manufacturer (EVGRA/MSI/Gigabyte/etc..) via they official web, they will guide u how to RMA their product........