Yeah theoretically a 2500K at 4.8GHz would match a 2600K at 4.0, but remember that the 2600K has hyper-threading, and no matter how far you overclock the 2500K, it's never gonna have 8 threads, so in multi-threaded applications the i7 will always come out on top.
I had board instabilities with my 2500K at around 4.5GHz using the cooler in my signature. the PC would boot, die, and then start up again. Removing/lowering the overclock to 4.3 seemed to sort it. I'm sure with better cooling you could get 4.5GHz with no problems, but my cooler only cost £20.