I'm going to throw this out there, someone will correct me if I miss the target. If the seats mentioned in the referenced thread are made on the OEM seat base, then the backrest position will be the same place and you'll have to do something there.
You missed the target.
When Russell rebuilds a seat using your own seat pan, they rebuild WHATEVER is needed for the seat to fit you. If they move you back by moving back the seat pocket, the backrest position will get moved back also. And the backrest will still be adjustable both vertically and somewhat horizontally too. And the backrest pad will swivel to allow you to "stretch" occasionally while riding.
Yes, this may cause less room for the passenger seat area if your sitting position is moved back. So that is something you and they can take into consideration if you routinely ride 2-up. But in most cases if they have the measurements for your regular passenger, they can rebuild the passenger portion of the seat to accommodate them easily too, and your passenger will be more comfortable along with you.
But if you ride solo like I and many other riders do, passenger space is never an issue. The entire portion of your rider seat will be TOTALLY rebuilt to your "rider" specifications (measurements) for your comfort.
A Russell seat also isn't just "adding gel packs", etc. Their seats create an entire "pocket" for your butt that is also surrounded on 3 sides with additional support areas (some people call them "wings") that specifically spread out pressures and provide additional support for the sides of your butt and your upper thighs.
You get what you are willing to pay for. My last Russell got 85,000 riding miles in 5 years before my accident. I just sold it to another rider in Florida who still has the same kind of motorcycle I had the Russell for. It still looks brand new too 'cos I took care of it. Next week I have a ride-in appointment at Russell again to get my new Spyder RTL seat. I plan on riding it the same way...long distance touring and lots and lots of comfortable miles. And the entire seat, including the backrest position, will be "moved" to fit me when it's done in ways that just adding gel packs could never do.
Yes, they are expensive, but you get what you pay for.
As an aside: They will most likely be moving my seat pocket FORWARD a bit based on my previous Russell seat. Which will move me a bit closer to the handlebars. Which is why I won't need to spend darn near $1000 for a Tri-Axis bar replacement with labor installation. Adding just "gel packs" to my OEM seat would never do that.