In general most sloop rigged cats (w/o kite) do not have a backstay. Cats need a stiff mast to keep their headstay straight.

49ers do not have a rotating mast. The more you derotate the mast, reduce draft, the more the mast bends on the minor axis. Bending on the minor axis increases gust response. When adjusting 12 to 1 downhauls though not automatic work quite well by compressing the mast resulting in flatter main (opens slot) and twists the leech for the gusts. Modern cats shift gears for each wind velocity (light, med, high) quite well, thought not automatic. So if you tapper the mast for one wind range, it will lack speed in another. ie. a tappered mast for heavy air, will not be stiff enough for med air in waves. One could find a compromise here.
As I recall it is a real art to tappering an aluminum mast, and adds a lot of dineros to the cost.

The Tornado class tappers it's mast at the top, but I think they are going to a stiffer, with the advent of dbl trap, more SA, & kite.? Tornado mast design changes all the time. I would really be interested in reading about it's evolution.

I would imagine the A Class and 18HT class would benefit the most from a tappered mast at the top for gust response, because it is no longer concerned about headstay sag.