I don't believe the website has specific specification delivery estimates - all colour/wheel/interior/autopilot combinations on the SR+ show September as an estimate but I highly doubt that is the case if you choose the most obscure combination.Even the tesla custom order page is showing white SR+ (any exterior colour) with no added extras available for delivery in Sept. The LR and Performance models are definitely more niche as they're showing as Nov delivery.
They have either over produced SR+ vehicles for this quarter or they just aren't as constrained as the LR and P models. Certainly the batteries in the SR+ aren't a constraint, people in the US are now being offered the lower mileage range SR+ that we get in the UK to avoid delivery delays, so the batteries (or even the vehicles) are coming from China and not the US.