I just spoke to a PAL Instructor, and explained to him opinions in this thread. His reply was that "just because a magazine of a different caliber can hold more rounds of another caliber, that does make the magazine illegal but it also does not make it legal to load it with more then 10 rounds. The law is simple and that is what we teach, all handguns magazines are restricted to 10 rounds maximum loaded, whether more can go in or not. It is capacity specific not caliber specific and just because a 10 round .40 cal mag will hold 13 rounds of 9mm, that does not make it legal to do. If this was ok, do you not think we would teach it that way?"
I guess we all have our opinions and ways of reading the law, but personally I would not want to get caught doing this and then go bankrupt trying to prove I am right while my tax money is paying for the pitbull crown attorney that works for the CFO who is trying to bury me and getting paid for it.