As far as LCD mounting is concerned, it seems like the in-dash option is certainly the best. If you wanted to squeeze the HVAC controls down (and make a new bezel) you might fit a 7" in. If you relocate the HVAC controls to the center console (I've seen it once) you could get a nice sized LCD in there.
A roof mounted one blocks the rearview mirror... The shifter takes up a lot of the center divider space. I don't want sunvisor LCDs... I think I do want a H/U setup...
I've got a 2000 Trans Am, and I'm going to see If I can get the screen where the factory radio is. If, not maybe I can make a pod that mounts where the TCS switch and cigarette lighter are in front of the shifter. Here's a pic of a guy's Firehawk that re-molded the radio bezel for a single-DIN radio. There looks like there's enough room there for a 7" LCD.
Here's a pic of my car with the stock dash, for comparison: