No issues here doc. Im at 20k - gearbox is as smooth as ever, even better now with the slipper clutch converstion. Rob who is on here and on our uk forum has had an issue with 4th on his 2015 bike but, thinks it was due to his healtech qs. Mines got a HM QS on there and has been on for over 4 years...i dont use it all the time but when i do its when im pushing it so if anyone is asking for a failure id say its me.... When the 4th gear went on Robs (dont quote me) but he had it apart and back together inside a day, with no prior knowledge of stripping the bike down which IMO is testament to how easy these bikes are to work on. He pulled the motor, turned it upside down, removed the sump and then the bottom half of the crankcases getting access to the gearbox. I think he bought a second hand gearbox which was nowhere near the prices your quoting. I think without looking at his videos it was 18 months ago and hes not had an issue since. Personally, if i get a gearbox issue ill have no issue pulling the motor and doing it myself. Id almost be happy to as the bike has been totally fault-less in 6 years of ownership.just search on this site
But it is on hundreds of Google sites
If you don't have it, Hallelujah.
If you do, the parts are only $600-800 but the repair time is a minimum 26 hours