Which sportboat to choose

Yes they did: the "Action 24" which was dual-spreader with a masthead kite. They buillt two and sold one. A year later the boat was sold and restored to the class rig.

Personally I don't think the 707 wants "supercharging". It provides big-fleet racing with a lot of mixed-sex crews. A more powerful rig means heavy all-male crews will get all the chocolates (which is fairly true even now). The 707 provides a lot of fun to a lot of people as it is; leave it alone.