Sorry for the delayed post - haven't looked closely at the board in a while.
I did the Kaplans early on, probably a month before the test. After that, I had plenty of other more "representative" tests to work on. My opinion, and others may disagree, is that while Kaplans seem harder, I couldn't put a finger on why - somewhat more ambiguous verbal content? Just "different"? So I personally had a hard time figuring out how these "harder" questions would improve my score on the real thing.
Timing might have been the biggest diff. I ran out of time on each verbal Kaplan. I had less of an issue with other tests partly because the verbals were "clearer" to me and partly because I just got better at them. I think that even after the process, the Kaplan verbals would stress me more on time though.
I did Kaplans under worse than real the first time and better than real the second time and got about the same scores. First time, I was in an open area at work where there was some din and hub-bub (even though it was evening). Second time, I decided to seat myself in a nice conference room and had plenty of space to stretch out, nice wall art, etc... you won't get that in the real gmat!