Most everything is there at diff 7, but there are a very few things in diff 8 as well.
But unlike a lot of RTS games, it's not simply that this is a "cheating AI" above that point, or something. Since it is so much apples and oranges between you and the AI, the difficulty is just tuned to what "feels right" at the various levels, since it's not going through the same resource gathering, etc, like you are. I think the general consensus is that it's considered "fair" through diff 8, and 9 and 10 are semantically the "cheating" ones. But it's all semantics, of course.
The main thing is that you won't be missing out on much in the way of AI tactics if you play on diff 7, and that's by design so that peopl can get the fun and adaptive AI without it being bloody hard all the time, heh. The stuff that diff 8 adds is pretty small, and centered around what the AI does in a few edge cases, if I recall.
