phantom, by the numbers, I don't know that it's that much, if you compare both rounds at their best. The WM will hit harder in close, but it seldom seems that people are worried about that.
A .30-06 can be loaded pretty hot with a 180 grain bullet, though a WM can be pushed about 200 fps faster, still.
To be fair, compare hot hunting loads, not Garand-compatible loads or even regular factory loads. These numbers are NOT what you will see in a standard factory-load chart. Both .300 and .30-06 are usually loaded slower than what I'm going to quote.
Hornady sells hot hunting loads commercially, and similar loads are in most reloading charts, as well. To make it easy, I'll use the Hornady commercial ballistics.
The .30-06 180 Grain Hornady Light Magnum is rated 2900 FPS/3361 Ft-Lb. at the muzzle, and 1959/1533 at 500 yards.
The .300 WM 180 Grain Hornady Heavy Magnum is rated 3100/3840 at the muzzle and 2063/1701 at 500 yards. That's just about 100 fps faster than the .30-06 load. I don't think that will buy you another 100 yards with the same effectiveness.
(.300 Wby Mag is only a hair hotter than that, probably no real discernible difference from the above .300 WM load.)
For comparison, a standard factory load .30-06 180 from Hornady is rated 2700/2913 at the muzzle and 1751/1225 at 500 yards. You gain more range by going from a regular .30-06 to a hot load, than you do by going from a hot .30-06 to a hot .300 WM.
Not even considering recoil and barrel wear, these rounds get exponentially more expensive with each little increase in velocity. You decide if it's worth it to you.
http://www.hornady.com/shop/?page=shop/browse&category_id=9b21ee3d3362d53196e45f77a2b066c9