Re: What caliber for maximum downrange velocity?
Move down to the ballistics section of the forum and read every thread in the first 10 pages.. All you have mentioned (as well as the others) will do 1000yds easy. So it seems your big concern weold be game at 600yds with energy. Again, I think any of the ones mentioned are more than enough with a properly placed shot.
If you just want a new caliber, that is cool. If you want to do more LR precision, look at the 6.5 family of cartridges. Great from 100yds to 1500yds and light on the shoulder. The .260 has similar ballistics as the 300WM/WSM out to 1k. But since you already have a 300WSM, get a Savage and then you can reload for two rifles. Just build you LRP 300WSM with a longer/heavier barrel and more ergo stock. That being said the 30/06 is no slouch either.. see what MontanaMarine has done with his.. over a mile I believe (at altitude though).
Actually here is what I would do.. get a Savage LA in 30/06, the get the 300WSM bolt head, nut wrench and a target style barrel in 300WSM. Now you can shoot both calibers with the a quick 10min barrel and bolt head swap. The LA will be nice for the WSM so you can seat your bullets out long. Just my suggestion. Heck, order a .260 barrel too, you have the bolt face already (30/06).
Good luck.