I've got nearly 50k miles on my Kawasaki accessory seat--bought it not too long after the bike. I had been sanding away slopped spots of the OEM seat, but the foam is just too spongy leading to sinking into the seat as @twowheeladdict mentions--and, I only weigh about 155# plus riding gear. So, anyway...there was some getting used to the accessory seat. I just wanted to ride and not dwell on the seat too much.
So, with a few rides other sensations began to get my attention. Buzzy bars and the location themselves. Took about 10k miles and a year to get the bar location sorted, and even added 5/8" spacers just last year.
Meanwhile, my Kawasaki accessory gel seat developed a divot where I had been sitting. This past summer I opened up the seat to see what could be done. I've added three layers of some perforated, grippy, spongy material. Pulled-down the cover and re-stapled. Working pretty nice.
I see no need to replace the seat, and if I decided to, I'd probably be wondering where to go like you are now. Having had a Corbin on my 2K8Z1K, I understand the firmness (hardness) you are referring about; so, I would probably really consider another Kawasaki accessory seat based on my experience with the C14 Kawasaki accessory seat. :2cents: