I would think there´s a reasonable chance that end of Sept would be OK, it depends on the weather really doesn´t it.
It´s high, I´d recomend acclimatisation, it´ll certainly not do any harm. Radson seems to have been sensible. If he did the normal way via the Gouter up Mont Blanc he´d have been nicely acclimatised for the more technical Mat. (just guessing though)