Teddy & I were fortunate enough to get Lens very first Limited Eddition SpyderFest special RT-S #1 of 5 when he first decided to build them back in 2011. They came with what was then the 5 most sought after after-market mods preinstalled. Those included:
Elka Shocks
Utopia Backrest
KewlMetal Highway Pegs
HMT 3rd Brake Light
Custom Paint from Danny Farris
Now after living with the bike for almost 2 years and over 34,000 miles as a daily driver and also 3 long distance trips, I'll give you my opinion, as you asked.
The improvement of the Elka Shocks over the stock shocks was dramatic. However, with that said, we recently added BaJaRons new Anti-Sway-Bar and I have to tell you, WOW!!! I find myself learning that I need to adjust my ryding style, as the improvement is that much better. If I were in your position, I'd first recommend adding the RonBar and see how you like that improvement before shelling out the $$ on Elkas. Please, don't misunderstand, the Elkas are worth every penny, but I honestly believe for the average ryder, the RonBar would be all that you'd need for superior handling. If not, then you could always add them next.
On long road trips, even with having to stop every 125-150 miles to fuel up, you'll want some form of adjustability for your knees and back, otherwise you will get sore and tense & that makes for an uncomfortable ryde. The Utopia Backrest is worth every penny & is a high quality product. But there's also many who love the Corbin seats, as you can get them with the optional backrest as well, not to mention the fuel door, and also optional heat as well. A bit pricey, but most that have them would never go back. There's also Russel Day Longs among others, you just NEED a backrest for sure. But the backrest is only half the adjustability equation, as you also need some type of highway peg or floor board to be able to move your feet around and be able to put your knees in different positions. Lamonts NBV pegs are very popular along with John's Seal Floor Board, again, many options to choose, but you will NEED something.
The HMT 3rd brake light, while not NEEDED, it does add much more rear visibility at eye level from the rear & I personally wouldn't be without one.
The paint was an added bonus and not NEEDED for a long road trip.
Besides the above, We've added a Garmin Zumo 665 with added XM & Weather & its proved invaluable on many a road trip, you can ask ArntUfun & I think he'd agree. The XM is good for long road trips, as the RT's radio is not really user friendly when traveling out of your home area & a pain to keep adjusting for clarity while travelling, IMHO. Its nice also to have all our MP3s on the installed SD card as well, cause a road trip with your choice of tunes is even better.
Hope this helps, & I'll add more if I can think of them.
ENJOY THE RYDE