well they will not keyhole, keyholeing is from under stabilizing, you cannot overstabilize a bullet if it has the strength to keep its jacket. may not shoot as well as a heavier bullet that is the optimum weight for that twist.
I also don't believe you can slow it down enough to spin the same speed as it will from a 12 twist and get any kind of performance.
at 3500 fps in a 12 twist the 40 gr spins 210,000 RPM's, with an 8 twist you would have to back the load back to around 2325 fps to get 210,000 RPM's
RR