Doesnt the 2600 have turbo boost too?? Maybe you are talking about the overclocking potential of the 2500K. Not sure I really understand what you are saying.
Yes they do but with the 2500K, you could likely run it at around 4.5Ghz and it's going to take a lot of slower HT cores to match the performance AND it costs about $75 less than the 2600 and only $10 more than the 2500. There are some other differences and you do need to investigate those, but for most of us, they don't matter.