In reply to Jon Stewart:
Be optimistic - early September is definitely not the peak of the midge season, and midges have been relatively scarce, at least in the Torridon area, this year.
Predicting what you will experience in September is impossible, as I'm sure you know. If you get calm, warm and humid conditions you will undoubtedly experience some misery. But equally you might have breezes or frosts and an almost midgeless trip. In any case, unlike June, at least it gets dark in September, so you will definitely experience some relief in which you can cook and eat burka-free.
The two worst and utterly unforgettable midgings of my life - a life with a very generous distribution of midgings - both took place outside (and then inside) my tent on the 6th June (20 years apart). September has never been too bad.
Maybe consider early spring for a future trip? The first two weeks of May are often warm, dry and (almost) guaranteed to be entirely midge free - the loveliest time of year in the west highlands for me.
Have a great trip.