Not sure what would be needed. But it is probably best to keep roadmap as unmodified as possible to be able to track new releases. Gpsapp is smaller and I might just grab some or all of your changes at some point.

Sounds fine, not many changes to GPSApp are really needed for this anyways. Although I may have to look into getting the zoom scales to match, not sure what that would involve.
That's one of my issues with the data format. I had already decided that the data should probably be stored in several 'layers' so that at a larger zoom we don't have to seek past the unused details. i.e. one file with interstates and possibly borders/large bodies of water for the whole of the US. Other highways and possibly mayor roads in per-state files, etc.
The author has this in his TODO file as something that needs to be done so hopefully in the near future this will get fixed.