Combat trim does not center the ball for you. You bank and you'll still slip. It just trims the rudder, as pilots would do (either via trim settings or pushing a little harder on the pedals -- remember real world doesn't "spring to center" like USB joystick controls do)
As good as the AH flight model is (best you can find so far!!) it does have certain observable lackings here or there. Mind you, not too many. It is still by far the best WW2 flight sim of them all.
One of them is this apparent lack or torque. HiTech says the torque is fully modeled, but something "dulls" it IMO. Compared to previous versions of Aces High, we have less noticable torque effect causing spin-outs on planes at slow speed turns, and on runways, and doing ground loops.
The P-51B used to be far better at turn fighting than the P-51D because it had the razorback and the spin was less likely to happen in tight stall fights. This was back in AH1, before the change. Now, it seems something has changed.
HT says the Torque is still there, so I can only say "something else" has caused it not to be as effective. Some underlying variable or code process makes the end result not the same as in real life.
Of course, this in no way helps HTC figure out what the problem is (if any) and resolve it, so there's nothing I can do about it.