This was a big change with 7th ed.
DS vehicule now arrives at combat speed (0-6") instead of cruising speed (6-12") as it was in 6th ed.
So yes, passengers can now fire at full BS. That's what makes Raider w/ splinter rack better than before... and "yes" Medusa can use their template from an open top vehicule
You can disembark, but you cannot assault, because passengers are arriving from reserve this turn.