Chaosmancer
Legend
you forgot to add that new GWM adds +1 STR, that would make your primary stat 18 and not 17 at the given level or 20 vs 19.
So old GWM must use default hit rate of 55% and 1 less base damage.
Unless you start with 16 / 16 instead of 17 / 15 and the feat brings you to 17/16 as your top two stats instead of 18/15
I tend to not account for the strength mod, because I feel like it comes with too many extra steps and caveats.