I had the chain conversion installed at the dealer, then I later swapped it back to the belt myself.
I swapped it because the chain was pretty noisy and I wanted more experience with the silent belt.
Chain driven it sounds kind of like a razor scooter on steroids.
Acceleration was quicker with the chain, I'm pretty sure I did a very light wheelie through an intersection.
The speedometer gets it's figure from the motor controller, the bike is limited to 85mph, and that's 85mph on the dash, so technically speaking the bike had a lower top speed regardless of the dash. My dealer did NOT re-program the speedometer, so when I swapped back to the belt everything worked as normal again.
When I swapped back to the belt the noise difference was really prominent, IMO the slower acceleration is negligible because that electric motor really puts in the work. It is still definitely a noticeable difference in acceleration, but either way I hit my top speed pretty quickly. If you're into wheelies the chain will help quite a bit.
My intention for the chain conversion was to do more off-roading with the bike, but so far I mainly use it for commuting. I figure I'll run the belt until it breaks then swap over to the chain again, but I'm sure I'll change my mind again when that time comes.
Here's some threads with information that you might find useful:
https://www.electricmotorcycleforum.com/boards/index.php?topic=11018.0https://www.electricmotorcycleforum.com/boards/index.php?topic=11954.0IIRC there's a video on youtube where someone rode with a belt and again with a chain on their FXS. You can clearly hear the noise differences.