The only issue with 11' ceilings is a lift other than that when you start heating it you'll be great-full for the 11' ceilings. So If I were you I wouldn't raise the roof on all of it.Unless it needs a roof than I'd get away from the flat roof if your gonna do it all.
I'm assuming thats why you don't like the ceiling height. Good luck with your adventure.
Depending on the lift you put in, 11' may not be that much of an issue. You can cut the concrete and install a pit, or just do a local roof raise where the lift(s) are installed with windows to open for letting heat out in the summer.
You laugh. It took 95 Gallons!! 70+ of that on the ceiling to cover 60 years of black. I'm hoping to use half as much on the whole rest of the building.