In a 140gr class bullet the 6.5x284 comparatively to the 6.5 Creedmoore will get you about 200-250fps more at the muzzle. Respectively, with the same bullets such as the 140gr ELD from Hornady with the hottest loads the 284 will have a good bit more energy and, with a good bullet makeup, will have increased penetration on a bone hit.

Now, whitetail aren't armor plated or anything and they really aren't that hard to kill with proper bullet placement. But, with long range performance in the mix, your placement with the smaller 120-140gr bullets would really need to be off the shoulder which takes a pretty good bit of your target area out of the mix. RR takes tons of deer at over 500 yards quite often with smaller bore rifles and will attest to that I'm sure. I personally don't shoot game past 400 (sometimes if I'm feeling mean and I'm on a particularly rowdy hog I'll take it to 500) but that's an ethics limit I set on myself not because of lack of skill but out of respect for the animal and the things that can happen between trigger break and bullet impact. Accurately punching paper and ethically taking animals are 2 very different practices and skill sets and you should learn all you can about BOTH before taking long range shots on live game.

Know your bullets terminal ballistics. If you have a tough bullet that won't blow up at close ranges, a lot of times it won't open at all at long range speeds. You need to know all the numbers going in so you can figure your longest shot that your bullet will reliably open and not just pencil through leaving little internal damage and no blood trail to speak of. That's one of the things a LOT of folks tend to forget about when they take that 700 yard poke at an animal and then they wonder why they didn't find the damn thing.