The 7mm STW is, for practical purposes, a 7mm-300WinMag. (The STW is a necked down 8m Remington Magnum, the parent case of which is the 375 H&H. The 300 H&H is a necked down 375 H&H, and the WinMag family is based upon the 300 H&H case.)
If, OTOH, by "wm" you mean Weatherby Magnum, then the 7mm-300 Weatherby Magnum already exists, under the name of 7mm Weatherby Magnum.
Both rounds, for whatever it is worth, are fast, flat, and overbore. They are both suitable for North American and African non-dangerous game hunting out to any reasonable distance, and both will kill a barrel faster than most other cartridges, if you are lucky enough to be able to shoot that much.
And of most of us, either shooting game at the typical 100-300 yards, or spending the whole of the rifle's life at the range, there is no advantage other than for our egos. Which is perfectly valid, of course.