I barely noticed it, I regularly drive my wife's RHD car and would jump between the two without issue. You just sort of get use to it after a few weeks.
This is how I was with mine, makes little change.
I find the switch between my M3 and Mustang more so, because one is AUTO and one is manual.
So sometimes in the Mustang I am grabbing for a paddle shifter on the steering wheel when one does not exist.
Gains on Mustang is about right. A remap alone is around 330BHP on UK fuel. The CAI kits on the S197 give good improvements as the S197 has a pre-cat in the air intake with a CAI deletes, hence a solid 20-30BHP there as well, though same can be achieved more or less just by removing the pre-cat and using a panel filter.
Fitting long tube headers would get the car to around 375-385BHP, this is what I did with mine, I then supercharged it and ended up at 500+.
You will miss the uniqueness of the Mustang and its like on rails handling on smooth roads, but you will love the dynamics, comfort, interior, more power, better build quality and need I go on off the BMW.