Hey Pete, water's been high in the north this year but it's too early to say what to expect for Sept. So much could change in the next month+. I've paddled the Steel twice, both mid-summer, and ground out on swifts at times but certainly not enough that I thought it was a bad idea to go in low water. And some rapids will be boney of course and you'll be better to port.
You may be familiar with the hydrometric gauges from Water Office but here's the Steel if not. You can zoom out to see past months/year for reference:
https://wateroffice.ec.gc.ca/report/rea ... tn=02BA006Compared to last year, current levels are similar.
There are some general guidelines for Superior you may have heard like "prepare to be windbound 1/5 days in mid-summer and 1/3 days early and later in the season." This guideline will vary depending on who you talk to but I think it's fair.
I've only lost a full day on Superior three times, but you really should aim to be on the water at sunrise or earlier (bring easy breakfasts), and end your day early too. Paddling a calm Superior at sunrise is the best paddling experience I know, so it's very much worth the early rise.
The Steel is probably still my favourite river, favourite loop, and Superior is my favourite lake, so I think you'll have the trip of a lifetime. Unless there's a historic drought in August, I wouldn't change your plans for the Steel. It's a nice river to wade anyway with gravel and clear golden water.