Just a word of caution. The F3L will give you a different seating position from your RTL; however, that does not automatically also translate into no numb butt - the stock seat on the F3L can also be a pain in your butt. I wound up buying a 2017 F3L because my hips just could not tolerate being in the more upright sitting position of the RTL. I could barely stand 15 seconds of sitting on an RTL in the dealer's show room. But I did make myself test ride it (with passenger) for 20-30 minutes before deciding it wasn't going to work - I could barely lift my right leg to use the brake. Then, I did the same on the F3L - the legs forward position gave more comfort, allowed my hips to relax, made it possible to use the brake (rather necessary). Riding the F3L was tolerable, but the stock seat gave both heat and soreness after 45 minutes. The sitting position was improved, but the 'comfort' level was questionable. Others here have experienced similar outcomes. I bought an AirHawk pad; that doubled my riding tolerance to about 90 minutes. Not bad, but I wanted longer. So I invested in an Ultimate seat, and found I could go for 3 hours plus. Even managed to ride through a whole tank of gas without a break (250 miles). Lately, though, sciatic pain has increased to point where I went back and had the stock seat custom-built to a firmer, higher position; this has removed a lot of pain and added time and distance to my rides before needing to stop.
In short, I believe that you should be aware that sitting 'position' is one thing; seat 'comfort' is quite another. They are not necessarily the same, and switching from one 'position' to the other will not necessarily result in seat 'comfort' improvement. Go ahead and try out the difference in the relative ergonomics of the F3L versus the RTL, but don't necessarily expect that seat comfort will also change. You may well enjoy the F3L position, but you might also then wind up swapping out the stock seat that comes with it. That means added cost - I know, 'cuz I've now paid for two expensive customized replacements. Anything to keep me on the road, and on my butt happy on my Spyder.
Good luck to you in your search for butt happiness!!!
PS: Perhaps I should add that a few months ago, I had the privilege of test riding the 2020 RTL; what a machine!!! Sitting position is now closer to that of the F3 - slightly more legs forward than the older RT's. And the seat comfort is much improved, along with those gloriously large floorboards. My 30 minute test ride has me thinking of swapping my F3L and moving up 3 years in time. If you're seriously considering a change in Spyders, I'd suggest taking out one of the new RTL's as well as the F3L. The latest version is an immense improvement and has found great acceptance among buyers.
