I think the tire size difference may be the issue, not the traction control. One way to test this is to borrow a set of "square" tires to try on your car.
Your existing front has a roll-out of 78.6 inches, and the rear 79.5 inches. Technically this would be "Under-slip", or a negative slip value. I'm not sure how the traction control map is set up with MS45.1, but on race cars we use a different slip allowance for corners than in a straight line. The reason is that the side loading on the tire reduces the longitudinal grip resulting in say a 5% rate of slip being optimal in a corner versus say 15% when going in a straight line. BMW may have this mapped based on torque output from the engine, steering angle as well as rate of change of steering angle, G force, and finally wheel slip as a feedback mechanism. Changing the torque output value, or allowed value, may help you hide the difference in wheel speed causing the feedback loop to trigger an error, but it won't give you optimal traction control.
If you don't have access to another set of wheels, I would consider changing your front tire to a 235/40/18. (79.8" roll out)