It really comes down to you need a .600 BC with a muzzle velocity from 2900-3200 fps. We can debate this, if you like, but that is my opinion.
Nonsense. It is nice to have, but you don't "need" either of those, particularly the velocity. All that will do is shorten your barrel life.
A higher BC bullet will not make you a better long range shooter, or help you better read the wind. What it will do is lessen the consequences caused by reading the wind (and compensating with hold or a sight adjustment) wrong.
Reading the wind is really all that matters in long range shooting, and all the big magnums or hot 6.5mm rounds will do for you is be a little more forgiving when you mess up your wind calls.
For example, shooting at 500 yards, when you call the wind is 60º at 8mph, but it is really 90º at 12, how bad do you miss?
At 500 yards with a 6.5 (140 SMK @ 2800) you are off by 1.5 MOA, which is just under 8 inches. A couple of extreme examples for comparison (which I have wind dope sheets worked up for) are the 30-06 150 FMJ, and .223 55gr FMJ. For the same error, the .30-06 will be off by 2.3 MOA (about a foot) and the .223 will be off by 3.1 MOA (~16").
Using the MR-65 500 yard target, if you held and broke a perfect shot, the 6.5 would still be in the 10 ring. The .30-06 would be in the 9 ring, and the .223 would be in the 8 ring.
Also, for "long range" target shooting, 500 yards is just the beginning.
If you get the wind right, the result is the same in any event, a hit.