Fast Twist is geared to heavier bullets
A 1:8 twist barrel was not designed for light Varmint bullets. It supports heavier bullets 55 - 77 grains. "Varmint" rifles used to generally have 1:12 or 1:14 twists. But Savage for example, now only makes 1:9 twist 223.
Having said that, I shoot several 1:9 twist Savage rifles, and I have been successful loading Barnes Varmint Grenades in 36 and 50 grains. Using AA 2200 velocities were around 4000 fps. I was able to strike steel plates with loud report 100-200 yards. I also shoot Nosler 40 grain Ballistic Tips with the same result (WC844). None of these bullets ever flew apart in a cloud of red dust, as some thought they would.
Stabilizing Long and Heavy bullets requires faster twists, and there is a Stability Calculator here:
http://jbmballistics.com/cgi-bin/jbmstab-5.1.cgi . It answers the question of
Can I Stabilize a Given Bullet? (usually applied to Long and Heavy bullets).
But if you turn that around, and ask
What Will Lighter Bullets Do? Other than the supposition that too rapid spinning will cause self destruction (I have not experienced that), nor do I have any means to quantify that. So, I think you have to try it and see what happens.
I just looked up 10X and TAC data on the powder maker's online load data, and there is plenty of data for those powder and bullets 55 grn. and lighter.