The charts here say that it should like the 45grain bullets the best but Sierra/ Berger says 75-80grain? Or have I misread this info?
The charts should give you an idea of what a given twist will stabilize and what it won't.
Don't try to analyze the specific group sizes, that's not going to provide useful information. All the graphs show is acceptable accuracy (say under 3" or so) which means that the bullets were stabilized, or unacceptable accuracy (over 3") which means that the bullets weren't stabilized.
I've added a note to the post to help explain.
Generally speaking, if we're talking about conventional lead-core, copper-jacketed bullets, 1:8 twist should stabilize bullets up to 77 grains, maybe a little heavier--at least out to 100 yards which is the distance used in the testing.
If you're talking about monolithic copper bullets, steel core bullets, or any other bullets that are unusually long for their weight then you may have to go lighter (or to a faster twist) in order to get proper stabilization. Also, if you stretch the range, have a very short barrel (resulting in low muzzle velocity and therefore slower spin), are shooting very light loads (again low muzzle velocity/slower spin), or are shooting in very cold conditions (which lowers the muzzle velocity/slows spin), a bullet that is marginally stabilized under normal conditions may not stabilize.
If a rifle stabilizes a given weight bullet, it should also stabilize lighter bullets, although it may spin them faster than is ideal. That shouldn't cause problems unless the bullets are poor quality in which case the faster spin may amplify any imbalance issues/inconsistencies or in the extreme case where the bullets actually come apart from being spun too fast.