My 24" 284 is a plus 3, and it's way overgassed. For a 26" or 28" 6.5prc, I would be ordering a +4 or +5. I don't like uppers that require lowers that have non-standard buffers.
My 28" rifle +1 7.62x39 is slightly overgassed with a carbine buffer. ARs just don't need that much gas.