Just a quick update, I did fix the bug in my code - turns out it does change the win percentages somewhat. But the most interesting thing? The duelist/Shield master has almost double the number of wins in my scenarios. That doesn't necessarily mean anything because the mix of opponents is going to throw things off, just I'm not so sure the GWM is that overpowered. Plain old sword-and-board seems to hang in there just fine thank you very much. Even for monsters where GWM won, in some cases Duelist/Shield master was quite close. For example if fighting a troll you probably want GWM. A wood woad? Technically GWM is better, but only by 1.5% at 10th level.
Also, I did some scenarios with slightly higher AC, 10 Dex and so on and the relative numbers didn't really change much.
Anyway, the overall best result for the different styles was 14 for Shield, 8 for Great Weapon, 4 for Two Weapon. I'll probably play around with a few more options and do a longer post at some point.