I'd like to see a test of that rather than just an assertion. One thing we know about deflection is that lighter weight produces lower deflection, and if the shaft is narrower it should be lighter weight. There's a reason the low deflection shafts started coming out with narrower diameters. I can't see any reason why the larger diameter would be lower deflection.
I doubt you'll see an actual scientific test anytime soon between cf shafts on a robot.
As for assertion, well, if your ever around a 12.4 and a 12.9 at the same time, it's really, really easy to test.
Put ob at center of end rail, put cb on spot. Line up for center hit and then shift over either left or right and shoot.
The above will give you your answers "if" you have a straight stroke. Then again, if your stroke isn't straight or you "think" it is but, it isn't, well....lol....then you'll only "think" you know but, wont.
These are the people that has told me they think the 12.9 is a "little" lower:
Danny Smith
Josh Roberts
Alex P.
There have been several others to say the same thing but, I'm not sure you would know them.
Now, are the three I mentioned robots? Nope, but, they are about as close to one as I have access to.
Like I said, do the test yourself and then you won't have to take an assertion.
Let us know what you find,
Jeff