My thinking was, as incorrect as it might be, was that the 270 Winchester gives approximately 50 fps per grain of powder using H4831 as an example. The 270 WSM gives approximately 48 fps per grain of H4831. The gain of 200 fps (3200fps for the 270WSM vs. 3000 for the .270W) is achieved using 7 grains more powder, so the increase is producing only approximately 28 fps per gain of H4831, thus the diminishing return on gains of powder used. This refers to effeciency only.
If you're looking at getting to work every day, then maybe a Volkswagon is the most effecient choice, now if you want to get there fast and money is no object, then how 'bout a 'vette.
That was my understanding of overbore, but I am open to being educated about the subject, as I don't claim to be an expert.

Bobby, I think you are correct that it is less efficient. I also am no expert, and have never seen a definition of the term, much less an agreed upon definition. I would say the gains would have to be approaching zero for the term overbore to be applied.

Mathmatically, to define a curve that is not linear, we need at least 3 data points. And for the velocity (y) vs capacity (x)curve to bend to a slope of zero gain, that curve is not linear.

Divide the case capacity in grs of water by the bore squared in inches. When you get a value of 1000 of more it is overbore.

One can also use this numer to determine optimal bbl length. Just multiply it times the velocity of the bullet you plan to use and raise this number to the 0.22 power to get bbl length in inches. Once you get over 26 inches, you are getting into overbore territory.