In highly CPU dependent processes like multimedia encoding, it will be close the the clock increase ratio (3.3GHz->4.5GHz = 35%; 3.3->5GHz = 50%). In games it would be less, but it will vary from one game to another. Still for 15$ more (~7%), it's hard to justify buying the non-K.
Also of note is the K version has the HD3000, ie: all 12 EUs are enabled. While it won't matter for gaming since you should have a good discrete card, it shines in video encoding and such. It'll be significantly faster for that. You can find the recent Tom's article where they compare it.