I have the Ultimate seat. I do notice that I do not get as tired riding with that seat as I did with the OEM seat. It is a well-made seat and the details, i.e. the stapling etc. is of higher quality. Mine has heat and that is a nice thing to have. There are other differences in how the seat works on top of how it is made.
Overall, the stock seat compared to the Kenda tires, less than stellar shocks, and totally inadequate sway bar, is not too bad! I think BRP did a passable job on the stock seat in terms of looks, durability, and comfort; but the Ultimate seat is definitely a step up in terms of materials, again the seat detail work, and the pan. The pan is firmer and of higher quality than the OEM seat. In terms of overall looks, the Ultimate seat blows the OEM seat out of the water. I have the white ostrich on mine with the red/black Spider on it, and it really sets that bike apart.
Is it worth 2K? That's a lot of money for a seat upgrade. I would say that maybe the jury is still out on the price/value question, and I do question my sanity at times, but I judge most things by asking "would I do it again?" And I have to say to that, "Yes, I would!"
In terms of the Corbin, I know they make a fine product, but I do not have any direct experience with that seat.