At 800 yards, a 300 gives you over 1000ft/lbs of energy more. Everything I have ever been taught is that you want a minimum of 1400 ft/lbs for an elk. Will a 6.5 creed still work, absolutely. However, I wouldn't do it. That bullet only has about 1000ft/lbs of energy at that distance.
Is the "rule of thumb" of 1400ft/lbs just made up nonsense? You have much more experience than me on this stuff. What is your energy cutoff? Or, is energy irrelevant?
Ft-lbs energy is a useless metric for determine “killing”. The depth of the wound, width of the wound, overall shape of the wound, and what organs are damaged are what determines “killing”.
The bigger a bullet is, the more tissue it “can” destroy. Doesn’t mean it will, but is possible. Case in point- “mountain goats are really tough”. “I need a 30cal magnum and a tough bullet”. The 30cal magnum part might have increaed the potential wound size, however the “tough bullet” decreased its actual size, I.E.- a 30 cal Barnes TSX has a significantly smaller wound channel than a 135gr Berger from a 6.5.... BUT both have plenty of penetration for a goat. In that instance, even though a “magnum” was used, it will consistently kill slower than the smaller 6.5... It’s all about the bullet.
Given adequate penetration, the wider a wound the more tissue it damages and the faster it kills. As long as the bullet had enough remaining velocity at impact, it will kill fine. With most loads, between 750 and 800 yards is where most LR bullets will drop below the required velocity in the Creedmoor.
Yes, a 215gr Berger 30cal can, and will kill faster all else being equal to a 130gr Berger. The potential wound channel can be bigger due to more bullet and fragments. However, the 130gr kills well and killing effectivley is so much more than what cartridge is used. To consistently, and repeatedly kill is a combination of shot placement, terminal ballistics, and external ballistics. Those all have so much more to do than how “big” a round is. Once you have adequate terminal ballistics than really it’s about maximizing your hit rate. THAT is where the smaller cartridges come into their own.