often wondered this myself, we only ever had one 6.0 in europe I think that it was what you call the stock 6.0. Not really sure what the 'upgrade' was apart from taller mast.

The 6 will always be competitive upwind, we were faster on that point than the i20s when they 1st arrived.

The 6 with a kite will always struggle against a cat that was purpose built for a kite. assuming that what you mean by a '20' class is spinnaker sailing.