IF there was a change to the mustang, it was a good while ago. I noted many months ago a similar situation, where I over-trimmed ("FUBAR-trimmed") with the mustang easier than I did before.
I honestly figured it was due to me maybe using trim more agressively and actively than i used to when I more frequently flew the mustang in AH, but still... I don't feel as if it used to be that unfriendly. I'm not convinced anything has been changed though, I used to not use trim as proactively, and I trim out dmage much earlier than I used to, where I would only start using trim after slowing down and loosing enough control with just my stick. I've assumed in my case that trimming out damage earlier, and when you're traveling faster with CT enabled, leads to FUBAR-tim settings when your mustang slows down to limping-home-speed.