Can you please be more specific about which Kaplan tests you are taking? Are they on a CD? are they part of the online program? I just finished a Kaplan course (on-site) and took 7 of the 9 online CAT tests. I found them to be very representative of my actual score, and types of questions. Verbal was slighly harder then the real test.
Some of the older tests from the CDs and online were aparently much harder, but this seemed to be corrected in 2010. They were approx 100 points below what you could expect. Since then, the more recent tests are much more representative. Here are my tests (all Kaplan):
Diag: 570
CAT 2 - 600
CAT 3 - 640
CAT 4 - 620
CAT 5 - 680
CAT 6 - 650
CAT 7 - 690
Actual GMAT - 710
Worrying will not help your score, in fact, being positive and feeling confident will really help you. Keep studying. Every hour you put in could cover that one topic, or reinforce that one concept that could bump you up. 3-4 more questions right on g-day could put you at 700! 630 is strong, your not far away. Best of luck!
PS - A 5 on AWA is all you need. It is generally accepted that anything over a 4 is great, and will not give you any added benefit on g-day. Don't worry about the essay b/c it sounds like you are at an acceptable level.