I don't anything about the Kilmartin sills, but I do think it's worth getting sills made of thicker sheet metal than original. Those inner sill are pretty important from a structural standpoint. I got a set of heavier gauge sills from Healey Surgeons, but I don't think there was as much price differential as between the Moss and Kilmartin ones. (Of course, that could be because the thinner ones were more expensive!).