The above answers are correct regarding the twist rate.
Velocity fariations (between book and your rifle) like your speaking of have more to do with barrel to barrel variation, powder lot to lot variation, chamber to chamber variation, and the sum thereof, than they have to do with the change in twist rate.

not to mention match barrels can vary by manufacturer as well as by which number barrel it was on the button when rifled, etc with reguard to the depth of the lands and grooves. Also,..a 1:8 twist will exert more drag or jacket intrusion due to the increased rotation. The 1:8 twist will shine with the 140gr class of pills,...but if you don't wish to shoot them, the 1:9 or 1:10 twist would have probably been a better choice. I went with the 1:8 in my 6.5-06AI specifically to shoot the high BC 140gr class of bergers, sierras, clinch rivers, a-max etc.

If the greenhill formula is correct, then my Hornady 129 sst's require a 1 in 7.94 twist to stabalize. T=(150x[.264 squared])divided by 1.316.....Rojelio