I've had my 89 Bandit for just over a week and found the seat very slippery. Had a Bandit Ltd many years ago. That had a textured seat as stock, and I don't recall any slipping problems solo or two-up in the three years I owned it.
I'd look into getting the seats recovered with some more grippy material. Triboseat is a name I've seen mentioned a lot lately which might do the job. No personal experience with it though.