Yep. Gainwards clock very well. My Ti500 (Ti 550) is rock solid at 265/602. I can get quite a bit higher, but it crashes under extreme stress, or shows artifacts in 3D rendering.
You may have gotten that high, but have you stress tested that clock setting in D3D and GL? If not, it's meaningless.
Clock it up that high and run 3DMark2001. Then run a GL program. If it can complete a couple hours of both, it's a valid OC.