Another factor that I have heard discussed in the "low density - slow powder" argument is the physical shape of the propellent grains, and the potential for detonation to take place. Examination of some damaged weapons have, as I understood, shown evidence of metal flow as a result of detonation.

One theory is that a compacted mass of propellent, loose in the case is able to impact the base of the round with sufficient force to intiate a detonation. The argument is that this is possible with the greater cross section of the slower powder.

...another argument is that the grain structure is broken up by the impact, dramatically increasing the available burning surface.. this is similar to the "do not tumble live rounds in a cement mixer" story..

As you say - until we find a way of reproducing the effect in a lab, we are all speculating.. however I am quite clear that dragons lurk in that particular corner of the tables..