All fuels have a critical compression ratio, or the ratio at which the fuel and air mixture will spontaneously ignite due to compression generated heat. Many other factors come into play:
Is the fuel liquid or vapor? Liquid fuels may have different CCR due to heat absorption and latent vaporization.
What is the rate of compression (usually based on piston speed/engine rpm)?
You state that the piston quench is near perfect, for which fuel? Engine design engineers have been trying for 100 years to find the perfect combustion chambers, and are getting very close, finally, mainly due to computer modeling. The quench (squish) areas are generally used to initiate high speed air-fuel movement to accelerate the combustion process. The need for squish areas are different for gaseous fuels than liquid fuels.
As for propane, some engines can operate at 14:1 which is generally considered above propanes CCR of around 13.5:1, but other factors come into play, as camshaft events, ignition timing, cylinder head design and material and so on. As for what is the ideal compression ratio of propane? That depends. How long do you want it to last? I have seen engine with 375,000 miles at 8.5:1, and industrial engines at 12:1 at 30,000 hours. Of course, the usage and load cycling are different, but here is a case of an engine being optimized versus one being converted from a gasoline mode.
One last thing, propane is unique in that at different phases, it can change physical properties. Extremely high compression ratios can approach the super critical liquid point.
Franz