Comparing core clocks between different architectures means absolutely nothing(in this case Kepler vs Pitcairn), it's like saying
Oh this FX-8150 runs at 4GHz therefore it beats this i7-3930k running at 3.2GHz
Most 7970s run at 900-1000MHz yet still manage to beat 1GHz+ GTX670 and GTX680.
Also 7850 beats the GTX650Ti and once you factor overclocking and 13.2 drivers it can even equal a 660 at times.http://www.tomshardware.com/reviews/geforce-gtx-650-ti-benchmark-gk106,3318.html
the 7850 has a 256-bit vs 128-bit bus bandwidth meaning better AA/AF and high res gaming.