Just out of curiosity what do the Russell Day Long and the Sargent seats offer that the original does not, and how?
You could always find a local upholsterer to alter your existing seat if you know what it is you want, ie; harder, softer, taller, shorter, wider etc.
I've already had the original low seat done by Tony Archer of Huddersfield, he done a good job, but it's still not right "for me", although it's better than the original with more padding. I also have a BMW tall seat which I was looking at sending out to RDL for conversion. The difference is they will give you a money back guarantee, but they want to have a couple of goes to fix it if you not happy first time - the shipping is a killer, but it would still be better than the £800 I would lose if I bought a Sargent seat and didn't like it. RDL also build it to your shape, size and weight, no one else in the UK offers this, RDL are basically the gold standard of custom seats for touring, and as I've already tried three seats - a standard, a tall, and a custom made one by TA, then I'm kind of running out of options for long distance comfort
PS, I may just have a fat ass, but I'm losing weight to help with that also!