If you tweak your HT settings (overclocking, basically) and get a little more bandwidth to the CPU, you might get more of an improvement for the Thuban. If I recall, part of the issue with the six core chips was that the cores were all fighting for bandwidth and cache.
I agree that you should probably wait for Ivy Bridge, but if you want to overclock now the i2500k is an attractive choice. The 2600k really won't offer that much of an improvement vs the 2500k.
I could sound like a total noob, and I know tolerance for noobiness is very low these days - but would you recommend I turn off two of my cores? Will that improve performance?