Speed increase for electric Yamaha PTV

[Raptorbob]

My 2018 Yamaha PTV just came back form 3 days in the shop to get a speed code increase from 15MPH to around 20MPH. I was told that everything they tried was not successful and since the cart had a factory lift kit Yamaha had obviously locked out the controller to preclude a speed increase.
The shop was recommending installing speed gears in the diff. I asked about installing an Alltrax controller and was told that their experience was that some of the installations were causing flaky operation.
Does anyone have experience with Alltrax in a Yamaha DRIVE2 installation?
Gears, aftermarket controller or another Yamaha controller that would allow speed code installation. These are my choices, probably all of these will negate the factory warranty so I am looking for advice on the best choice.