There are also certain items that we can carry across back and forth. The main issue that Canadians have are with bodies/receivers/pistol frames and sometimes the slide as well.
If some of us are that picky about our kit, and we travel in a large enough group, there is bound to be someone with the skill to reassemble several guns in a night. For gearbox upgrades, we can just bring our own existing gearboxes which will be faster than having to get inside the gearbox to do a swap.
If it's complete conversion kits, all we would need on the US side is a metal receiver - although a complete gun building project like that will take some time and the builder could be limited to building about only 2 or 3 guns in one day.
Once the operation is complete, we can disassemble the guns and take back all our original parts, and in the end, we get our own customized guns and the cost to JOHNUSA is much lower.
That would be the route I would like to go if I went on this op, simply because all of my guns are so customized and would like to keep it that way if possible.