I see cars driving normally on space saver tires pretty regularly. I avoid driving around those cars. They clearly can go 65-70 mph. They do regularly around here. Their weight carrying capability is fine, I suspect the reason they recommend 50 mph is their traction for braking is quite limited. The stability system should keep you from spinning but it can't help your braking distance.
One reason I do not carry a plugging device, liquid or thread type is I want to have a chance of fixing the tire. The place that will install my next set of tires will patch, from the inside, even run flats. So if I do not muck the tire up, I may not need to buy a new one. Even if I have to stop and air up every half hour or so, the TPMS will keep reminding me to do it and I won't ruin the tire. I'll get home. I may regret my "solution" some day but I doubt it.
128i Convertible, MT, Alpine White, Black Top, Taupe Leatherette, Walnut, Sport
Ordered 5/22/09, Completed 6/4/09, At Port 6/9/09, On the Georgia Highway 6/13/09, Ship Arrived Charleston 6/24/09 at 10pm, PCD 7/21/09