Being in Ireland, I should not be speaking for my neighbours, but I will start the ball rolling and then they can come along and contradict me.

I believe that there are many more Spitfires than Stealths.
The Spitfires are better organised, with a proper racing circuit, a Nationals and Europeans.
I also believe that the Spitfire sailors as a group are some of the top cat sailors in the UK and give the F18s a run for their money. They have come from just about all the existing classes, Dart 18, Hurricane5.9, F18, Hobie 16 etc. Many of the events on the circuit are open events, sailed with other classes. Apart from John Pierce, you don't see the Stealth sailors featuring in the top positions, as you do the Spitfire sailors. The Spitfires have good numbers at events and get good racing against each other. They feel that they do not need to race F16.

This next bit is just my opinion.
The Dart Hawk class started very well. Many boats were sold all accross Europe, some of the top sailors were sailing them and they won many events. Then F18 took off, other classes developed. One rival paid all the top sailors they could find to sail and develop their cat. This cat started winning events and, of course people bought them. The Hawk builder could not afford to keep up, and few new boats were sold, even though it has been upgraded and is as fast as any of the F18s.
The Spitfire is in the same position. It is a very exciting "One Design" Class, selling well, and I believe that most of the sailors think that it should stay that way.

I do not know of any Spitfire sailors who race "one up", the class rules would not allow it. I am sure that some sail uni for fun on light wind days.
The only Stealth I raced against sailed uni. He was a good sailor (he has been on the podium with other classes)and the Stealth was fast, but his spinnaker was launched from a bag on the deck and even though the wind was light, launching and retrieving slowed him down. He was ahead of me many times, but I finished ahead of him on the water in two out of three races. His SCHRS handicap number also put him down the fleet. http://www.catamaran.ie/results/2002/InlandResult.htm