What you are referring to as a tensioner is not a tensioner. It is actually a belt damper. The damper is used to reduce belt vibration and it has some success in doing that. I put a damper on my bike but would not do it again. There have been reports of damper failures and when that happens, you can generally kiss your belt goodbye. Instead of a damper, just lower the belt tension as suggested above. Most people report good results by doing this. And, besides, it is less expensive to just adjust. Good luck..... Jim
Now that I've got 8000+ mile on my 2018, I notice belt vibration a lot less - and I'd think that would agree with a lessening of belt tension over time. I really only notice it when at 75mph+ AND a crosswind, otherwise it's not there.
I'd be really concerned about the bearing in a dampener - consider the speed of the belt multiplied by the small diameter of the bearing and the fact that such bearing is out there in the shizz... I wouldn't trust one longer than a season unless you babied that tensioner bearing daily...