Enough?? Maybe not, at least as important as compression is available battery power. Batteries at 0F have lost something like 60% of their power at compared to 75F. After an agressive round or two of 125amp glow plug cycling there is not much left to acutally run the starter.

Remember the primary job of a glow plug is NOT to heat the combustion chamber, its to provide a red-hot surface that will cause small droplets of fuel to ignite when heat-of-compression alone is not sufficient to get things burning.

IHMO Glow systems, compression and battery power are the big three of cold starting - I agree its hard to pick "the" most important issue.