Neat - that seems to be a big problem for C96 owners - finding ammo or components.
Personally I've come across at least one local story of C96's not liking hot loads and telling the owner in a very direct way, so I'd be leery. That said, if we haven't heard about someone in the states messing themselves up or doing horrible things to them then it should be pretty idiot proof.
Very neat indeed. I can take partial blame for that as I harrassed him for enough boxes of ammo that it was worth ordering the crate.

I'd say keep ordering the ammo from him to keep it worth his while to have a steady supply of 7.63 ammo in Canada.
Now even though I agree not to shoot hot loads in the C96, what I would be most concerned about and get done first is a full spring replacement. Just for safety sake the mainspring and bolt spring are required to be in good shape, and since you're in there you should replace the firing pin spring as well. The only springs that don't directly affect the safety of the gun are the mag spring and the trigger spring.
I've heard some guys say "it looks good" or "it strikes the primer hard enough to ignite" but the mainspring is crucial to the safety of yourself and your antique investment. I know from experience that a mainspring replacement is frustrating to say the least, but it's absolutely necessary. It serves two purposes in the C96, one is to drop the hammer with enough force to ignite the primer, and the other and most crucial is to provide a cushion for the bolt before it hits the bolt stop. With a weak mainspring over time the bolt stop will crack from firing any ammo, hot or light loads. I would suspect that a weak mainspring is to blame for a lot of the "hot loads broke my C96" reports.
The easier spring to replace is the bolt spring. This serves the obvious purpose of returning the bolt to firing position and as a cushion for the bolt before it hits the bolt stop.
The firing pin spring doesn't seem like a necessity for safety, but think about it, if you just replaced the mainspring to a full power spring the hammer will be striking the primer with enough force to nearly go right through the primer. This at the very least can cause the firing pin to break prematurely, and since those things are serialized a replacement will mess up a matching antique.
I know how much of a pain in the ass it is to replace the mainspring in the lockwork, and a lesser pain to get into the antique bolt, but to me replacing the springs are a necessity above not using hot ammo.