The 300 WSM is the real deal. I don't see the advantage with the 270 WSM, or the 7mm WSM and especially the 325 WSM.
It is possible to get 3000 fps from a 270 Win. with 150's if you have with a 24" or longer barrel, and 2900 fps is not difficult even from a 22" barrel. The best 270 WSM loads I've ever found are only 50-75 fps faster than the best 270 loads. I wouldn't choose a 270 WSM over a standard 270 for such a small gain.
I fell into a 300 WSM really by accident and have been wringing it out for the last few years. With 180's in my 30-06 about 2800 fps is the best I can do and still get good accuracy and be within what I consider safe loads. I've seen guys claim another 50-60 fps., but not with book loads. With pretty mild loadings I'm getting 2950-3000 fps with the WSM and have worked up book loads at 3100 fps. Some factory loads chronographed 3100 fps. My best accuracy however is a bit under 3000 fps, so call it about 200-250 fps better than 30-06 and no more than 50 fps slower than 300 WM.
The biggest selling point to me of the WSM is reduced recoil. I can come within 50 fps of 300 WM, but with about 12-15 gr less powder. If you do the math that 50 fps along with less powder means recoil about 1/2 way between 30-06 and 300 WM. From the same weight rifles my 30-06 load has 22 ft lbs of recoil, the 300 WSM about 26 ft lbs and a 300 WM about 30 ft lbs.
Do I need it? No, I don't. In fact I'm seriously considering selling the 300 WSM. Not because I don't like the round, or the rifle, I do. But in reality one of my 308's is most likely all the gun I'll ever need. My 30-06 is really more gun than I really need and I'll probably just use it as my big gun.
Winchester borrowed/stole the idea of the 300 WSM from Rick Jamison who developed it as a Wildcat round. It works as designed. For whatever reason the same concept in other calibers just hasn't worked out as well.
Many studies have been done on this subject. results are always the same a shorter fatter cartridge is always more accurate then it's counter part
This is also why they tend to recoil less too, more efficient powder burns means less powder needed as well as more accuracy. You get the same when comparing 308 to 30-06. The new 1000 yard benchrest world record is currently held by a 300 WSM.