While the rim diameters are nominally different, my experience has shown that US made brass (Win and Rem, for example) are actually formed with diameters the same as .308, .30-'06, etc.
Past 3000fps? Depends on the bullet. Experience (again) has shown that pushing the 6.5 heavies (139-142gr) beyond 2850fps is generally counterproductive; precisely because it leads to early throat demise. The 6.5-284 shooters already know this, and my limited recollection on this suggests they get about 1200 good rounds out of theirs.
Pushing velocities is not a positive strategy. Regardless of velocities, they all get pushed around by wind, and a bad wind call will generate a bad POI regardless of velocity. Better to learn wind skills, and give that throat the benefit of your increased knowledge. The critical muzzle velocity is the one that will reliably deliver the bullet ontarget above the supersonic threshold, preferably 1300fps or so. More than that could be excessive.
These days, 1Kyd has become a common yardstick, but this is probably an artificial standard that has arisen long after cartridges like the Swede were first introduced as Sniper implements. Realistically, Sniper engagements generally take place at widely varying distances, typically unknown to any real precision, and generally well below the 1Kyd yardstick distance.
If you can't reach your target reliably supersonic without generating max loads, you are most likely using a cartridge with too little case capacity and/or too short a barrel. This reflects the admonition about being sure to bring enough gun. Going to the other extreme is just as bad, as heavy recoil can be counterproductive to the volume of practice that proper familiarization necessitates.
For example, my 28" .260 Rem was competitive, but not dominant, at 1Kyd.
For 1Kyd and beyond, my preferred chambering would be a .280 Rem (AKA 7mm Rem Express). It has enough case capacity to generate the needed velocities without generating throat churning temps and pressures. It also employs a commercially available case. Hobbling it with a barrel length of less than 26" is not useful, and 28" would be better; better for getting the full velocity benefit from that case capacity with needing to generate those wasteful temps and pressures.
Greg