A significant degree of impact damage would be far too open to abuse, not to mention removing a significant amount of skill from PvP.
As to vehicle dmg/time; whether it is above or below a solo player's output is largely irrelevant so long as balance is maintained.
One problem is the assumption of the number of people active in the deployment of a vehicle, and thus where the balancing of a vehicle should lie. 2 people? 3 people? should a rhino be equal to any other two man team (be it apu/ppu, pe/tank or whatever)?
More thoughts when I am a bit more sober....