So, to help people in the future, I made it to Stahlstrasse in Essen a couple of times and can comment as follows:
it does seem to be a bit of a BDSM / domina hub.
There are probably about 100-150 windows in a nice compact, hidden street, about 10 minutes walk from the centre, and 2 minutes from Berliner Platz, and about half of them were operational.
Lots of Latinos, Africans, and a few EEs, none of them really to my taste, and I would say not really any fantastic lookers.
What was great for me, and lovers of domme, dominas, Mistresses, and BDSM, is that there were about 10-15 windows with Mistresses, and that lovely RLD sight of red and black leather, PVC, chains, whips etc.
Most of them were constantly occupied, and I understand it is a good idea to grab their phone numbers, and book in advance, but I managed to grab a great hour (12.30 in the early hours) with a German domina Lady Jasmin.
I would recommend her and this street for anyone into the same stuff as me, but maybe not make it the only focus of a trip.