Affects assist cutout point, not actual power/boost (I guess indirectly it sorta does). The difference can be substantial, actually. Range of 3-7% depending on tires. So a full MPH difference on average (5% difference).
I'm payin' for 20 MPH top end, I want 20, not 18.5 and certainly not 18. ;-) Where I really noticed this was in an eMTB race - those on non-Shimano systems had a 1-2 MPH advantage in the open sections and though I pedaled my ass off (HR was 156 average for the race!) I still was at a disadvantage.
Not super noticeable on everyday rides, but I did notice the cutoff on places like transfer stages or section of doubletrack/road.
Either way, it should not require a visit to the dealer and the cost/inconvenience. For that I reason, I bought a STunlocker license so I can set it myself. And every time I use it, I do worry about bricking my drive unit!