I've killed over 130 whitetails with mine in the past 25 years...I would guess 8-10 were in the 275 to 300 yard range....I guess it would do in a pinch...
I have 3 buddies that have retired their '06s in the past 5 years...They both thought the .243 was too "marginal" when they started hunting with me 15 or so years ago....after helping skin a couple of dozen deer shot with my .243 they kinda changed their mind....

One thing I have noticed....they shoot more than they used to...kinda enjoy going to the range more....grab the .243 for a little groundhog/crow/coyote hunting in the off season....I wonder if this is why they seem to make better shots when a deer presents itself........

A .243 will kill a deer just as fast as a .270...if you put the bullet in the heart/lung/shoulder area....One of my brothers has 2-.270s...one for him, one for his son...I have helped skin and track dozens of deer killed with both...Both kill quickly if hit properly....If you plan to gut shoot them...go with a larger cartridge.