I can second some of what Wayne said and also what you are seeing. I have close to 4000 miles on mine on a 650.
When I got it in the instructions said it was setup for a 650 so I put it on and started riding. Yep it was bumpy and figured that was just what it was going to be,
After listening to a number of people and with my rear tire wearing in the middle I decided to go with a car tire and the Ikon shocks.
Just put them on in the last couple of weeks and it has made a world of difference. One thing it does do is raise the rear of the bike. I did not reajust the wheels on the tow pac down so it is now more like it is probably supposed to be designed for. Litle tires hold it up for when it might fall and not as a trike. This does make taking corners a little differentbut not as much as I would have thought.
That idea came from Gramps in another thread. Thanks Gramps..
Also had the problem of the fenders bouncing up against my saddle bags. Didn't notice that at all this morning.
You might try raising the wheels some on both sides and just see. Walked out and looked at mine and I can take my toe while standing up and rotate them now so they are not down as far as they were before.
Don't know how much of this is like a 400 but that is my experiance.
Also when we were in EurikaSprings at the Renegade Rally last year there wer both types of scooters there so if you get a chance you might want to go there.
Good luck and hope you find a good solution.