Again, that all depends on what you're looking to boot. If the R-Pi doesn't need to boot a GUI you could shave some time there too. I think the best solution here is to have a battery pack that the R-Pi runs directly off of, and then you use the switched power that would normally power the comp and run it be the charge source for the battery. This way you get two major benefits. First, you can use the device for an extended period with the car effectively off. Second, it acts as a backup/power filter. If your car briefly loses power (typical when stalling/restarting the car with a manual), the comp doesn't get tripped, and if you experience power fluctuations typical from the alt, you can regulate that better.
That's sort of what I'm planning to do, but I haven't quite put it all together how it'll work yet. I'll likely have to do a lot of research for it. Moreover, I want mine to play host to a screen as well, which gets a lot more difficult, especially as regards power concerns.