Ya, I run all my stability at SL, standard, just to be sure, as I do hunt in even denser air than that condition.I have 125s and 135s but have not have a chance to try them yet. At my altitude of 3300', I'll need 3300 FPS to get SG of 1.5 with 1:8." At sea level, you'll need a ridiculous velocity, i.e., 4900 FPS for an SG of 1.5. I was using Berger's twist calculator.
Have you considered 122 Cayuga? https://patriotvalleyarms.com/flm-6-5mm-122gr-cayuga-hunting-bullets-50ct/. It is meant for 1:8" twist. Good luck!
Very nice. They told me to start at .050 and work in/out from there. Looks like .015 is going to be best for you.