There would be as much work involved getting the angles right on a 30-sided polygon as curving each stone to a set pattern, though quite how the curve could be translated from a 30th of a circle with primitive measuring techniques and sloping ground is beyond my 20th-Century brain...but certainly the overall effect would be more impressive with a curved arc than multi-sided.
Yes, that's pretty amazing when you think about it. I suppose it's not definitive but that sort of attention to detail (curved lintels) might suggest that Stonehenge was conceived and constructed where it now stands - not brought from elsewhere. Not sure if there's a great deal of logic in that observation however - I can still see it once sitting on top of Silbury quite nicely :-) 