As far as the STW or RUM having more capacity than the .338 Lapua case - wrong. The STW comes in around 97gr H2O, the RUM around 107-108gr H2O, the .338 Lapua is around 114gr H2O. Though the STW and RUM at roughly 72mm case length are both longer than the Lapua which 70mm case length which may give the visual impression that they have more capacity. However, you must remember that the STW is .532 case body, the RUM is .550 I believe, and the Lapua the fattest at .590. Hence, the Lapua is a bit shorter than those two but makes up for it in case body diameter.
As far as these cartridges are concerned, the longrangehunting forum will have much more experienced folks than this site. Mr Kirby Allen himself dwells over there along with Mr Shawn Carlock. As was said above, you should be able to see 3200+ fps with a long enough barrel with 180s in a 7 STW or 7 Dakota. Going any bigger than STW/Dakota capacity in a 7mm is really not going to net you very much unless you use a long enough barrel, push some high pressures, and run the absolute slowest of slow powders(US869 comes to mind or maybe give VV 20n29 a try or H-50BMG or the like).
You can most definitely get more velocity than the Short Mags will do, check out longrangehunting as I said. Its just up to you whether its worth it. The reason you dont read much of big magnums here is that a lot of shooters on this site are more focused on competition shooting or the like in which they`ll race through rounds in high volume fairly quickly compared to hunting; which is a good way to burn out a barrel if you are using a big ole magnum. Either that or they are just using what they see other people in the same game using (not demeaning in any way - using what the winners use is a good way to place your bets IMO).
Hunting is much different type of shooting than competition or varmint hunting. I dont see why you wouldnt be able to get 1000-1200 rounds out of a properly cared for 7mm STW barrel on a rifle used strictly for hunting. Including round count used in load development, a 7mm STW barrel or similar magnum chambering will net you quite the stack of harvested game or many years of hunting.
I used to think the big magnums like that were so cool and all the rage, but after actually shooting them.....no. As I`ve gotten older Ive realized accuracy comes first and foremost as long as the cartridge and bullet have the ability to reliably put the target down provided the proper shot placement. Not to mention, those big cartridges are just all around unpleasant to shoot: more recoil so a brake is recommended and then you must deal with the blast and noise. Plus, as you go up in case capacity for a given caliber, load development becomes more and more finicky due to such slow powders being necessary and hence more and more difficult to find extremely accurate loads. Why do you think, for example, the .308 and 6 BR among others have reputations for accuracy - the majority of loads out there shoot great.
IMO something around the .284 - .280AI - 7WSM range is about the best compromise for a 7mm. However, one thing Ive been trying to keep up with is some of the work "elkaholic" over on LRH has been doing with his 6.5 SAUM wildcat and 160gr Matrix VLD`s. According to him, hes getting 3050-3100 fps with the 160s, thats a pretty serious load ballistically speaking, and will really put the hammer down on elk from what Ive seen so far.
tl;dr Its all up to you what compromises you`ll make. You dont need to worry as much about barrel life with a strictly hunting rifle however you still have to deal with the recoil and noise or blast and noise - along with load development.
