On 10 Actual, I was climbing consistently up through the 160s. On 10 More and PrepTests from the last two years, I'm on a nice plateau in the low 170s (knock on wood).
Like you, I started with 10 Actual and have since moved on to 10 More and some of the preptests in the 39-42 range...and I haven't noticed a huge degree of difference. If anything, I'm more calm about the later tests and scoring better simply by virtue of having been at this longer than when I started 10 Actual -- I think maybe starting with 10 Actual sort of "ramps you up" to the harder tests. Then again, I know this hasn't been the case for everyone, so ymmv.
I'd say that if you're feeling *consistently* happy with your test-taking ability and your practice scores, this continues as you start hitting the newer tests, and you have some time free to put in this week, you might consider not delaying. Since as I recall you have up to the last minute to reschedule, I'd say just keep preparing and getting mentally ready as if you were going; that way, if your stride holds, you won't have spent this week flipping out with your indecision, and if it doesn't, you haven't really lost anything.