Same but different scenario with the transmissions- way more electronics and control modules are involved, far beyond unbolting the 4-speed and slapping in a 5-speed. I have asked, and was pretty convinced it's not worth the trouble. Was hoping it was more like putting an H-55f into a 60 series.
Where I wish I had 5 gears is when towing- it's a huge jump from 2nd to 3rd, and going up Vail pass or similar it's like you go from red line to nothing... you rap it out in 2nd then third is an RPM so low you can't maintain speed, so you end up shifting to 2nd again. I've resorted to just manually shifting to 2nd, keep it around 45-50, and forget about 3rd until I've hit the top.
I also wish I had 5 gears when wheeling descents- 2nd gear is just a little too fast for actual controlled downhills when it's just a little technical- think Ouray side of Imogene. So I either ride my brakes a lot in 2nd, or go pathetically slow in 1st. Seems like the 5 speed guys are more in the sweet spot of 2nd gear... just the right amount of speed, don't have to hit the brakes as much, etc.
82 fj40, 99 uzj100, 06 gx470
97 DR650, 05 525EXC, 01 XT225, 89 TW200, 09 TTR125, 07 TTR90, 07 CRF50