i have a 2014 bike and have never taken my bad ass dongle off it motor is fine and just lifts the 15mph limit, it does not give you any extra power tho and will drain the batt faster if pushing 25mph.
no dealer can prove a dongle has been used or posted any proof ever as martin fits them at ebike shop and honers the warranty.
using a dongle just makes the bike sclass so to void the warranty is just madness tbh as you are just using the motor what it was made for in the first place.
down side is the faster you go the less range you will get.