Personally I don't like the idea of inflatables for downwinding.. They are pretty wobbly.. pretty tippy.. and pretty sticky to the water making it hard to get planing.. and once you get going on a runner and need to step back to lift the nose and these boards do the opposite.. They do this strange reverse rocker thing actually dropping the nose rather than raising it.. I know punctures and leaks are rare on inflatables but I'd hate to be way out from shore in a strong wind and have any of those issues.. Also.. pearling is a big part of downwinding and if you pearl the nose of an inflatable you are likely to get shot out backwards..
I'm sure it's do-able.. but do-able and enjoy-able are very different things.. Doing light wind downbrezzes might be fun but serious downwinding is going to be very hard if not impossible.. I think SIC have an inflatable bullet that might be ok.. I might try my 12'x34" Glide Air one day and see if it's possible.. One day when I don't mind getting wet..