I'm also not sure how precise are the TS parameter numbers. If I have enough time someday, I may try to measure them. I do have a relatively good measurement microphone so technically it should be doable.
In any case, I'm extremely pleased by the performance and sound quality. In terms of the performance it's in a different league compared to my old 18" sub (on the picture on the left, B&C 18DS115-8)
I do have another such 21" driver and going to build another Skram soon.
I also have a 24 inch driver I bought on Alibaba, and will need to use hornresp to calculate the correct enclosure for it