How soon can we upload or download different tracks that others hace created?
Actually, one of the core features of that subsystem is that as soon as you connect to someones race they're running (or them, yours) it'll share it, and generate the lighting your end. Explicitly sharing the track-hosting amongst each others an additional step were still discussing ways and means of addressing. (Since you can name your track whatever, just having it overwrite is obviously not necessarily the best of notions, since that would wipe out your work if you connected to someone with the same named track, for instance)
as to:
It would be nice to see the ability to make the track lanes wider by having more styles of tiles to choose from. Maybe ones the are edge tiles and blank center tiles and different tiles for corners to make them more flowing so that not every corner on the track will be a 90 or 180 degree corner.
You'll find several additional .dif files that can all be placed after a general layout in
\client\data\interiors\racing
by going through the advanced editor
these include microtiles, and (I believe. will need to double-check) additional alternate arcs/arc-pieces (subnote: at time of writing, those are only for the stock RHG tile-theme. If enough folks show interest in the alternate themes, by all means we'll see about adding more of those as well).