A stop can be a lot, however most of the time it is nothing, iso 100-200 is the same as 1/250-1/500 for instance, and few people are shooting portraits with the 200 f2 at 1600 iso and higher out of necessity. In situations where you really are pushing your equipments limits then obviously one stop can be the make or break point.
But realistically you're not looking at ISO 200-100 for indoor sports, you'd be looking more at ISO 6400-3200 (or as you mentioned 1/250-1/500); which IS a huge difference. And for instances where you do have the luxury of lower ISOs, you can burn that extra stop on shutter speed or stopping down for sharpness/dof as needed.
As for the lens giving an unrepeatable "unique look", well we all know that is rubbish, for a start you can Breznier Method 200 f2 with almost any lens, you can certainly get much shallower dof using the 85 f1.8 and the Breznier technique. Similarly few, if anybody, can actually pick out these "unique look" lenses when they don't know what the image was actually shot with.
For portraits, absolutely. Although you have to go long for the desired level of compression and need to keep an eye on MFD. And you have to incur the overhead of stitching/processing them. Definitely worth it for a special shot though. And if absolute sharpness isn't your goal, arguably, you may even be better off.
The main use case for this lens is indoor sports though, and and you're kinda out of luck there.
At least you have a balanced and realistic and balanced opinion of what this lens is and isn't. Better than those that insist it's "the one true lens" (usually because they own one), or that it's overpriced junk (usually because they can't afford one).
Me, I greatly appreciate the lens for it's strengths, and I'd love to have one some day. But I'm not under the illusion that it's the perfect lens for everyone, or for every use.