I'd say at this point just get your 6Creed out there and see what it will do. You might be very surprised.
.308
The .308 would be a legitimate 1500 yd. round if there were bullets and twists made for them. The first step in the right direction is to get some Flatline bullets and see how well they do. The next is to get a tighter twist than 1-10".
FWIW, 338's and big .30's drift in the wind bad enough to make you miss too. Their rate of success isn't much better than the little rounds either for first round hits at 2k. You have to know the ballistics no matter what you shoot.
Just a quick observation - I am by no means an accomplished shooter, but really enjoy the challenge of long range shooting. I really appreciate the sandwarrior's insight in this thread. At the encouragement of many on this forum (including sandwarrior in another thread) I skipped the "big boy" calibers for now to enjoy extended shooting sessions at distances of up to 1800 yards. I posted a thread in this section of my results with a 26" DTA in 6.5 Creedmoor at 1,768 yards a week or so ago with 7/10 hits on a 30" plate with Hornady 147 ELD-M FACTORY ammunition moving just 2600 or so feet per second. Only one bullet key-holed the target and the drop called by BallisticsARC was dead on at 1768 yards at 22.3 Mils after truing velocity at 1000 yards. I easily spotted all 3 of my misses just off the right edge of the target and could spot every hit but the key-hole on light steel at that distance through a Vortex Razor AMG.
I just finished a ladder test of RL 17 which tightened from .25" CTC factory groups to one-hole, .023" CTC results that pushes the 143 ELD-X (have had a hard time tracking down the 147s until yesterday) at 2820-2875 fps and should hopefully reduce the vertical spread quite a bit and tighten the group at a mile. With similar velocity (expected to get about 2800 fps) and the 147s (which I'll have this Wednesday) BallisticsARC calls for 19.0 Mils at 1800 yards and 23.3 Mils at 2000 yards and still traveling at 1104 fps at 2000 yards. Granted, my DA is about 5500-6500 depending on conditions in my shooting areas.
I'd wager that some may think that what I did my first trip out to a mile was either impossible or a lie - but it's neither. While my rifle isn't the best tool for this particular application it seems that every caliber is a compromise in velocity, drift, recoil, barrel life, accuracy, etc - the list goes on and on. For me, I've settled on a 6.5 as a wonderfully capable compromise for ranges out to 2000 yards that I can enjoy shooting all day long. That said, I'll absolutely be adding either a 7mm or .300 magnum iteration in the future for all the reasons listed above - a better tool for the job, but at the possible premium of recoil and extended shooting sessions.
In short, I really enjoy doing things that other say I shouldn't be able to do. Sometimes I can and sometimes I can't - but I'll post a video of my results at a mile with my new loads with evidence to either result.
Try it and you might be surprised how consistent your results may be.