800-1000 words is sufficient for a 10, as long as it's a high quality essay. I can guarantee you that. The only reason longer essays can get better marks is because they go into more detail. So if you can condense the same level of detail into lesser words, power to you (makes for a better read, too).
1000-1200 is probably 'ideal', whatever that means.
For some perspective, my text response was probably between 1000-1200 and my LA was about 1300-1500. My context piece was ludicrously well over 2000 words because I spent an obscene amount of time on it (almost 90 minutes).
Guess which one was my lowest scoring piece? Context.
Essay length is not at all an indicator of quality! I've marked essays that have been 800-1000 words that are much better than ones between 1200-1700 words, and would get a much better grade as a result. It's not a concern at all. Your only concern if you're struggling to write under timed conditions should be to make sure your piece has a very clear intro and conclusion to make sure your argument ties together nice and cogently.