Saturno, it is common to see a person highlight a certain load/round when comparing to a larger caliber but they will rarely post the same hot load potential for the caliber they are comparing against.
I am a fan of the 280 and know it can be loaded to near 7RM but when you maximize the potential of the 7RM the difference reappears. Handloading will raise the potential of all calibers.
You are perfectly right...What I meant to say, I correct myself, is that you can reach common commercial loading performance for the 300 Win Mag.
Of course you can stretch the 300 WM too.
However not all cartridges can be maximized in the same way (because of the cartridge itself, the actions that fire them, etc...), some have more leeway than others....for example the 30-30 is pretty much "tight" you cannot stretch it that much, conversely, the 45-70 (even within the modern loadings segment) is extremely "upgradeable"