On my m6 I have had really good luck with the stock weights and a shift assist and cat orange spring in the secondary. I left one shim behind the secondary for alignment. This setup has worked well
I ride slightly lower elevation probably 5-8000 but i run stock wieghts and secondary but i like to run a cutler gold primary, a little higher engagement. the 600s are supposed to make peak hp at 7700 rpm so see where you are running with that and adjust your weights accordingly