What were the downsides of feeding them late September? Any at all?
I would say that varroa treatment is likely more important now such that over-wintering bees are from the least varroa-affected larvae.
Feeding for the winter is best delayed as late as practicable - but that will depend on your location and the weather, the latter being the problem to sort out reliably. Most feed earlier to be safe and sure that all will be filled, reduced and capped.
Also depends on how you intend to over-winter, one box or two or if you are prepared to dummy out any unused space (for heat retention) and if you are prepared to feed fondant in the later winter months.
Also depends on whether you wish to be administering any treatments too, (thymolised syrup, fumagillin, or other) as part of the feeding.
You could feed heavily now if there is a good area of brood, removing the feeder after a few days (as soon as the take-rate falls), so that the brood nest is not continually filled as bees emerge, thus leaving adequate space for late brooding of over-wintering bees; then top up later, if time and need.
As with most things about beekeeping there is not just one way to do it, as long as the required result is obtained.
I will not be feeding much, if any, because most of my broods are already well provisioned and I envisage there being little need for extra supplements of sugar. That situation could change, but I am more likely to unite and redistribute feed frames as necessary. That is just what I prefer; strong colonies into winter and hopefully strong colonies out at springtime. Does not always work out perfectly (lost one large full colony last winter, after the spell of snow, as they were seen flying both during the period the snow was laying, and afterwards) but generally seems a good method for my needs.
Hope this helps.
Regards, RAB