I've been issued the 200rd soft pouches, used them for a few weeks, and in the end considered them a liability and switched back to the boxes. As pointed out earlier, the rounds tend to fold and get dislodged from the link inside the bag when the weapon is placed on its bipod - a problem compounded by the telescopic/folding butt of the C9A2. When the weight of the C9 is distributed between the bag and the bipod, the rounds inside just get all twisted up.
A few solutions I've seen:
A teaser belt: Something I totally disagree with. It defeats the purpose of having an LMG at the section level because a 50rd belt hanging off the weapon is probably going to run out just as the riflemen are reloading, if not sooner. IMHO this isn't really a "solution", but I've mentioned it anyways because I've seen guys get away with it.
American 200rd Boxes: This were what I used. The American boxes have metal clips on the top of them, as opposed to the crappy Canadian ones. I managed to scrounge a couple of these, switched the rounds from the cdn box to the US ones, and saved the cdn boxes so when I eventually returned the ammo the Lot Numbers were still accurate. The US boxes lasted considerably longer without falling off the weapon without sacrificing reliability. A couple slips of cardboard cut out to the shape of the inside of the box cut down on noise, and I proved that this would not cause stoppages by inserting these cardboard pieces in the ammo boxes I used on the range.
A guntaped abortion of a 200rd soft pouch: A friend of mine realized that the reason the rounds were folding in the bag was that the dimensions of it caused the weight of the weapon to sit on the rounds when stood on its bipod. He managed to use guntape to fold the softpouch in a way that would cut down on its profile and essentially make a 200rd soft pouch into a 100rd soft pouch. I'm not sure how reliable this was.
A REAL 100rd soft pouch: The Recce Pl was issued these, and they seem to me like the best solution. If you have a 100rd pouch on your weapon, and 200rd boxes in your fighting order you should be able to provide the required level of firepower without sacrificing reliability or having to deal with that stupid ass box always falling off.
GO!!!'s elastic band technique seems like a good idea too, and more practical for training in Canada then anything I've mentioned.