I am thinking about Sargent or DP. Any other suggestions although I did find a French Company that makes solo seats with a storage compartment where the passenger sits and instead of a cowl.
That French seat is very nice, but also very expensive
Mookie has a point about suspension. Depending on your ambition, a well tuned suspension might be the best go-fast-mod.
If you get caught with a loud exhaust, it may end up with them taking it off the bike and destroying it. No second chances.
PM Raux. He's in Germany too and might know how much of a risk this is in reality.