Collected my car today 
My mp4 files that I play on my Mk4's Kenwood head unit can't be played on this Nav Pro
Done some tests, results:
It can't play mp4 files that are encoded in "high" or "main" profiles.
It can't play files 1920 x XXXX or 1280 x XXX (in mp4 or divx HD profiles)
It can play mp4 encoded in "baseline" profile at 720 x XXX
It can play divx Home Theatre profile (which is also 720 x XXX)
I believe the Nav Pro is 800 x 480, and tests on 800 x XXX on mp4's at baseline = failed
Only plays 720 x XXX at baseline for the mp4's.
My mp4 files that I play on my Mk4's Kenwood head unit can't be played on this Nav Pro
Done some tests, results:
It can't play mp4 files that are encoded in "high" or "main" profiles.
It can't play files 1920 x XXXX or 1280 x XXX (in mp4 or divx HD profiles)
It can play mp4 encoded in "baseline" profile at 720 x XXX
It can play divx Home Theatre profile (which is also 720 x XXX)
I believe the Nav Pro is 800 x 480, and tests on 800 x XXX on mp4's at baseline = failed
Only plays 720 x XXX at baseline for the mp4's.