Re: 260 rem or 6.5-06 on 700 Long action?
OP, have you thought any on the 6.5x55 AI? Stamped ready made Lapua brass, speeds very close to 284 case from what I am told, and likely better bbl life. It'll DO what your 260 AI is trying to do, with less pressure IMHO.
Otherwise, no reason not to just use the 6.5/284 as brass ready to go. I like to keep things simple myself.
OR you could buy a 264 Win Mag Sendero if that floats your boat.
Never heard anyone complain on any 6.5. If accurate, barrel life and brass forming are your two variables.
How much speed do you really need is perhaps your last question.