I dont think that 0.1 inch will make a diff either. its close enough that it wont mess up the ABS or traction control sensors.
I think thats why they give us the cheapo full size temp spares, the car would freak out with a donut spare on. it would think that tire has lost traction (it would spin faster that the others due to its tiny size) and the traction control would try to slow it down via braking so it revs the same as the full size ones. that would be cool to try on dirt, heh.. anyone wanna try it and make a vid?
so the next cheapest route is the full size temp spare - as cheap a one as they can get away with.
I do agree, a full matching tire/rim spare like my cherokee has would be nice. Im sure we could get a perfect match from the dealer but Im also sure we would pay through the nose.