Why doesn't it prove anything? It's better than a quarter mile, as it shows the car in its true form, after several laps, not just one top gear lap, they are all racing under the same track conditions, only real variable is the drivers, which is a big factor true, but I think they are all good enough on that track that there's not much in it between them.
I've driven an S2k and I thought it was mince, I don't like having to ring a car's neck just to get performance out of it, so yeah sure, its a 'proper sports car' in the right rev band. IMO it's still a step down :P