I get the impression, Mike, that you're a coaster nerd, so I will answer this with a straight-up coaster nerd answer. Hopefully you're familiar with the coasters I cite!
Roughness: SM is rough due to the elements, not the track itself, if you know what I mean. It doesn't jackhammer like, say, the Cyclone, or Cedar Point's Mean Streak. It's more of an elemental jarring. The best comparison I can come up with, weirdly, is Holiday World's Voyage. You know how Voyage is an awesome coaster *once you learn it* and can anticipate how to lean, when? But before that it's awful, because you'll just be sitting there and then WHAM you're going a different direction?
That's Space Mountain. Only, you can't anticipate and be an active rider, because it's in the dark. And even if you ride it a whole bunch, and get familiar with the layout, you have to lean the opposite way if you ride the other side! So you'll be riding along and suddenly the car is jerked out from under you -- left, or down, or right.
I think it would be the same with any wild mouse... if you can't see to anticipate the sudden drops and turns, it's going to hurt. I suspect that if SM were outside I would feel quite differently about it. I have zero trouble with BTMRR, but then, I am experienced at anticipating those elements.
The BEST advice I can give is to ride it first thing in the morning. We've found that it's an entirely different ride after the rails have cooled down overnight. (Please note that this does not apply over Christmas week when the parks are only closed for a couple of hours... ask me how I know :/) The more the rails warm up throughout the day, the faster - and therefore rougher - it runs. We can ride it first thing in the morning. It's an entirely different ride than at the end of the day.
Hope that helps
