Your i7-2500K is at the higher end. There are some better procs, the i7-3770K and i7-2700K are basically top-of-the-line. Some of the Xeon parts are even better - the E3-1290-2 looks like the very best/fastest proc you could run on a Z68. Your quad-core overclocked to 4.7GHz exceeds the spec of any other compatible processor, so you won't gain an "upgrade" unless your new part can overclock even higher.
Here's a summary of Socket LGA1155/H2 processors.
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