I would suggest that you start with a normal A-cat, and then add spi later on. They have successfully added spi on most brands of A-cats but if you are concerned about the structure of the boat I would recommend the Marstrom A-cat.

Mitch Booth took an Marstrom A-cat platform, added a bigger mast, bigger mainsail, double trapeze and 40 kg of led and raced it as an F18 ht. Nothing broke on the boat!

I've been sailing with spi on my A-cat for a while now and it is a real work out to sail with it. At 8 knots of wind you are trapezing both upwind and downwind. I haven't tried higher wind strengths yet but that will be interesting!

Unless you are a very experienced cat sailor I would recommend that you start without spi to learn the boat.

/hakan