Glips,
If you can manage 800 miles on the standard seat - then save your money and stick with it. I was getting numb bum after 80-100miles but the GEL seat's pretty much doubled that. The last time I did over 700 miles in a weekend on the standard seat it took 2 days for my backside to recover. AFAIK the cowl fits both Corbin and Sargent seats.
Albie deffo runs his S4 (Corbin) cowled on the rare occasion he can keep Caroline off the pillion
I was contemplating a Sargent seat for the trike as I prefer the shape to the Corbin, and had been corresponding with them via email on prices & delivery to UK. However Ducati's 1/2 price reduction on the GEL seat swung it in favour of that option.