RE: 270 wsm necked down to 243 or 25 cal?
I would think that, based on the case capacity, you'd reach the point of diminished returns at about .257cal. After that poing you'll be so over bored that to small increases in velocity you get are outweighed by the massive amounts (relatively speaking) of powder you'd be burning. I would think that a .264 WSM might still be efficient enough to be practical, but nobody'd buy it as it's a 6.5mm. For some reason the 6.5mm's just don't stand a chance in the U.S. market, why I don't really know.
All I can suggest is that you go ahead and try it. Make the wildcat and a rifle to shoot it in and see how it works. It might not do worth a crap, but it'd be fun to try if you've got a bunch of excess money laying around.