I believe the 6.9's and non turbo 7.3's are around 22.5:1 The aftermarket turbo outfitters do not recommend taking those motors above 15 psi, and I don't think most od their kits make more than 8-10.

Propane is not direct injected, it it injected into the intake before the turbo. I don't know how it would work as a starting aid, but I would assume the higher BTU content would make it easier to ignite. Probably a bad thing when the glow plugs are working.