as already indicated, I have done a conversion to 17". In contrary to all others here I will continue to use the 280mm brake disks in the front. The reason for this decision was, that I have already spend some money for the full floating disks not long ago. It also saves the money for the rest, means new calipers and so on...
The brake force is still more than enough, specially for normal road use. We are not riding a 180HP powered bike....

This is, how the adapter looks like:

A friend did the machining... Also we machined a new pivot for the front wheel, this saved me from changing towards other bearings and/or machining carrier of the bearing in the rim. Will show a picture of the pivot later. I am still working on the speedo drive. It has to be milled down a little, but the angle of the cable is a bit too flat for the (more narrow) 280 disk.
Will also show a picture later.
I have only powerpoints-files with drawings of the adapter and the pivot, if someone wants it...
Cheers
Felix
PS: I know that there are some screws missing, I just assembled it quickly for the fotos. I will not drive like shown here!!!