If you want to try 7-1/2's on pheasants, good hard shot is the way to go. It doesn't have to be plated (though that helps a little), but does have to be high-antimony shot.
I've killed a bunch of wild roosters with high-antimony #7 shot from a 28-gauge, and only 7/8 of an ounce drops them real well out to 40 yards. There are around 300 7's in an ounce of shot, so they're somewhere between 7-1/2's (@350 per ounce) and 6's (@225) in size. They're also the same size as the British #6, which is most popular over there for pheasant shooting--which is usually done on driven birds, so not as many are flying directly away, like flushed pheasants.
In fact I prefer 7's to American 6's for pheasants in the 28, because they penetrate pretty well but pattern density is higher. But don't see any real reason for them in a larger gauge, where a heavier shot charge can be used.
