My GTX 970 have the half performance. In 3DMark i have the same results like with my previous ATI Radeon HD 7870 XT. I tried 3DMark and Need for Speed Payback in Windows 8.1 (fresh install) and i had the same performance.
GTX 970 today:
GTX 970 when i bought it:
My previous ATI Radeon 7870 XT:
GTX 970 today:
GTX 970 when i bought it:
My previous ATI Radeon 7870 XT: