[MA70-3.0GT]

To be honest, the only reason you could have for wanting a donut spare is for weight saving right? I certainly hope you ain't going on looks!

Anyway, for the amount of weight you'll save over a full sized alloy wheel & the limitations (some would say danger) caused by the narrow tyre style, I'd say you'd be much better off just carrying 2 or 3 cans of tyre foam (it's called tyreweld here in the UK, think it's fix-a-flat in the US?) and/or a footpump.

At the end of the day, unless you're really unlucky you're not going to have more than say 1 or 2 flats every couple of years & most times just pumping them up will get you to a garage. It's up to you really, but I'd say either stick with a "proper" spare or if weight's that much of an issue don't have one at all.

EDIT-: if you wanted I suppose you could offset the weight of a proper spare by running only enough fuel in the tank for the journey you're doing, after all if you're concerned about a few lbs you're more likely doing short runs round a track or strip rather than the daily commute I'd guess...