The 1:8 is a compromise between 1:7 and 1:9. Keep in mind that what you gain in making the bullet spin faster, you are losing in forward speed. There is only so much energy in the propellant and if more of it is going into the rotational energy of the bullet, there is less to go into the translation energy.
The original M16 and M16A1 rifles were - if memory serves - !:12. That resulted in a 55 grain bullet going around 3200 feet per second with military rifle and ammon.