As much as I favor the sub gauge.....
To be candid, from a more broader perspective as far as Upland hunting is concerned - a 12ga is neither too much gun nor overkill for such utility. It is practically the most versatile shotgun.
The "appropriate" 12ga game gun could actually weigh anywhere from 6.5-7lbs and yet feel more like a 6lbs gun in terms of handling/balance etc.......I'm talking breechloading double guns mainly.
Moreover, as already mentioned, one could always tweak loads ranging from 7/8oz upwards and still obtain superior patterns overall when compared to a 16 or 20ga shotgun. A proper game gun is not really designed for use with heavy loads in the first place anyway. IMO, rather than the gauge its the type of loads used that could be viewed as overkill.
Admittedly, technology and mechanical design improvements have also made it possible for folks with different preference to choose from either a semi-auto or slide action shotgun in the somewhat similar 12ga bird gun category. The Benelli or Ithaca Ultralight specimens may be considered as examples.
Type of bore type selection criteria will also be highly dependent on the species of game bird(s), shooting/hunting conditions one may generally encounter. In ON, for example, one could easily make do with a 20 or 28ga (Wingshooting point of view)....the same will not apply for chasing birds in other parts of North America or the Globe.
Bottomline, an Upland Game Gun, in the true sense, will be built accordingly as such. Obviously, the same will also definitely be nowhere near the category of a 12ga Browning Citori or an 870 Wingmaster (just examples

) no matter how well "polished" the later firearms may be.