An extra 0.050 of freebore does not make a 200 fps difference, it would be closer to 30 fps.
Most likely a barrel with that much difference in velocity potential has other dimensional differences such as freebore diameter, body diameter, bore diameter, groove diameter, etc.
Aren't you having enough trouble with having to modify 5.56 mags to get your Blackout to work and the bad accuracy with supersonic loads to worry about how we designed our chambers that have been working for over 4 years?
The only round which I have developed that requires the mag to be modified is the 175 grain 330 Major PF 3-gun load. Otherwise, I like the 110-125 grain bullets - as they are both higher velocity and are proving the most accurate.
12 twist 6.8 barrels will stabilize a 130gr bullet, longest bullet usable in a 6.8 mag.
The neck on a 6.8 SPC case is 0.273 long. The Sierra 150 #1840 (1.250 long), when loaded to 2.283 OAL, is seated 0.657 deep. Which means it is protruding 0.384 into the case body.
A 9 twist 5.56 will not stabilize 80 gr 5.56 bullets or some will not stabilize 75s and 77s so there is a reason to use 8 twist barrels in a 5.56, there is no reason to use a 8 twist or 9 or 10 twist in a 6.8 unless someone like you wants to try to shoot a 150gr bullet from a 6.8 which would be retarded.
A 223 caliber Sierra 80 grain bullet is 1.090 long. When loaded in 5.56mm to 2.260 OAL, it protrudes 0.589 into the case. Since the neck is 0.203 long, the bullet is going past the neck into the body by 0.386 inches - MORE than the 150 grain 6.8 bullet!
So that raises the question - why do you consider it 'retarded' to go 0.384" into a 6.8 SPC body but important to be able to go 0.386" into a 5.56mm case?
An 80 grain 0.224 bullet is as long as one would ever want to use, and makes about as much sense as a 150 grain 0.277 bullet - also the longest one would ever want to use. They both have the same limiting disadvantage of starting to protrude into the case body by pretty much exactly the same amount.
Now let's calculate the Miller stability for the 80 grain in 5.56mm and 1:7 twist as the baseline:
80 grain Sierra 0.224: 1.090 OAL.
Lowest velocity it would be expected to go: 2400 fps.
Cold temp 0 degrees F.
Result: Stability is 1.65.
Now we ask.... what twist in 6.8 SPC would give that same stability factor with a Sierra 150?
150 grain Sierra 0.277: 1.250 OAL.
Lowest velocity it would be expected to go: 2100 fps.
Cold temp 0 degrees F.
Twist: 1:7 - stability 2.77
Twist: 1:8 - stability 2.12
**Twist: 1:9 - stability 1.68 **
Twist: 1:10 - stability 1.36
Twist: 1:11 - stability 1.12
Twist: 1:12 - stability 0.94
Bingo - we have a match. As you can see, if you like 1:7 twist for 5.56mm, then the closest to that in 6.8 SPC is 1:9 twist.
Likewise, 1:11 twist for 6.8 SPC is about like a 1:9 twist 223 AR. Not that there is anything wrong with that for most people/most uses... just less versatile.