The advantage a 6 Creed has is proper twist/chamber/mag box melding, from the get-go in every factory iteration. In short, it's launch was executed properly based upon available and projected projectile R&D. The 243Win is faster, but, with very few exceptions is twisted such that the speed is useless for projectiles over 100gn and the mag box isn't long enough to seat those projectiles out to their full potential anyway. Barrett Fieldcraft and NULA being a couple of the glaring examples of the 243Win done right.
A competent gunsmith can consult you to the proper twist for your chosen projectile and chamber any .243 bbl to 243Win such that it melds to whatever magazine option you've chosen and it will outrun the 6Creed. The 6Creed's advantage is being turnkey from your favorite retailer. No waiting on a correctly twisted barrel, extended mag box, custom stock for same, or a gunsmith to thread/chamber your barrel.
The litigious nature of our society means that neither the ammo nor the firearms manufacturers are about to go diddling with SAMMI specs that have been in place for decades. They'd rather offer you a "new" chambering before re-certifying an old chambering with a new twist rate.
