Don't make your time restrictions too harsh, you might get a really easy L.A. piece and a god awful context prompt. It's good to know what your strengths are, but be prepared to re-strategize (?) once you're in the exam. Even SACs can sometimes be a good challenge, forcing you to throw all your preparation out the window
Firstly, 850-950 is fine. Obviously quality>quantity, but it's generally hard to score really well with less than 800 words. By the end of the year, hopefully you'll have so much to write that you'll be well and truly over that limit anyway. (There's actually no official 'limit' though, just general advice based on what teachers/assessors have told me.) Don't worry about writing more in SACs, that's good practice for expressing all your ideas or tackling difficult prompts. Focus on content for now, it's easy to fine tune the word count towards the end of the year.
That said, if you're heading towards the 1200+ range, you might need to cut down. This isn't the case for everyone, (I think my T.R. essay would have been around that, and I know others who wrote even more) but ~900 should give you a rough guide as to how much VCAA want you to cover. They're not expecting a thesis, or a whole lot of background details. Maybe go through some of your SACs/practice essays and work out how much of your essay is critical analysis vs. how much is irrelevant waffle
If it's all good then just keep doing what you're doing, but if you find there are a few sentences or chunks of paragraphs that don't really accomplish much, then concentrate on whittling down your essay to the important bits. Assessors are more impressed with what you can do effectively in a short space than how much you can cover in an hour.
Re: timing. 15 minutes is probably too long for editing. Try to edit as you go rather than waste time at the end rereading the whole thing (especially if you're not a fast reader.) Just practice as many different article formats for L.A. and prompts for Context and T.R. so that very little will surprise you, and you'll give yourself the best chance at the end of the year