This might look like a good idea but the problem is that you're going to dissipate a lot of power in the regulator. For a Pi taking an amp it'll be dissipating 7V × 1A = 7W, which is a lot of power.
It's possible to use a switching DC to DC converter. However I'm unsure as to the quality of some of them, especially if one were foolish enough to buy a cheep, dodgy one from eBay
Finally, I'd keep motor power as far away from Pi power as is humanly possible. The nasty spikes coming from motors are likely to be very unpleasant for the poor Pi.
I'm sure other electronics fiends will be able to help out here....