What kind of balance do you look for? Do you expect a carbon gun to be balanced the way you like out of the box or will you add ballast if necessary?
I'd rather have it slightly negative. Don't mind tweaking it. In fact, adjustable ballast is a great idea... since there's a ton of difference between a 6.6, 7, & 7.5mm shaft.
My first carbon railgun (not carbon euro) was a RA 160cm CF Saracean - thing floated like a cork with the shaft in!