Well, if I remember correctly from a link someone else posted, the Ma Deuce ammo came in 50 round belts, so for the armorers, it would be easy to link 6 belts to come up with the 300 round number, whereas to link 5 standard belts, then a 6th with 17 rounds would be a little awkward, especially since they couldn't get an even number of 17 round belts out of a standard 50 round belt. (Of course, (2) 17 round belts and a single 16 round belt could). The 267 round limit probably came from some technical order that used the precise number to come to the design load of the ammo combined with the bombs or drops. Considering that the P-47N, for example, had hardpoints that could carry the 300 gallon drop tanks (single 2000+ lb load), I'd say that even with a 165 gallon tank (at about 1000 lbs), you could safely assume it could withstand the additional ammo without exceeding the design load. Of course, with the biggest drops, there was a +3 g limit. May have something to do with loading the hardpoint to some weight that allows the maximum design load for the hardest maneuvering. I don't know. Here's the limits from the P-47N POH...
Now, the technical order is necessary in order to preserve the manufacturer's contract liabilities with respect to combat performance--i.e. Republic guarantees the USAAF that the P-47 will perform as advertised as long as the specified weight limits are followed. Sort of like engine limitations that we often debate because some pilot said his crew chief changed the turbo regulator to allow him to pull 75" in his P-47D-5 or something.
As far as the anecdotes go, obviously most were about ground attack profiles, but I remember one, if I remember correctly, about a pilot in Italy having an overload ammo load for air-to-air action. I remember because I believe the aircraft was hastily configured, and the pilot didn't know what his convergence was set at, or how much ammo he had, since it wasn't his normal aircraft. Anyway, I'll see if I can't find it, but I can't really remember if it was from one of my books, or from a link someone else posted, or something on Mike Williams site.
You bring up a good issue Krusty, I'm just curious as HTC typically deliberately models the aircraft for specific ordnance configurations and I'd be curious as to see why they included the overload ammo packages for all of the Jug models. Not that they haven't made tweaks before, but they did it initially for a reason, rather than arbitrarily...