I'd run furmark or some other stress test program before downclocking. As you might not need to do so.
MSI afterburner, EVGA Precision and RivaTuner will allow you to downclock the card if there is a need. And downlocking it with one of those programs or any software will not void your warranty. If you flashed the cards bios, then that might be a problem if something went wrong with it afterwards.
Many thanks for this. I did run furmark for about five minutes when I first got the card. Even though the temperature stabilised, it was very high (close to 90 from what I recall) and scared the hell out of me so didn't let it run for longer. No issues though so maybe I'll leave well alone unless I get problems.