You can do a search on this forum as this issue has been debated to no resolve. I also own an 03 4.6iS and have performed everything from Dinan software to dealer diagnosis...the downshift issue was still present. I sold my Dinan S2 4.6iS and purchased a lower mileage 4.6iS and the issue was still present even after replacing trany and installing a new torque converter.

I know several people with 4.8iS' who also have this issue...i guess it's inherent to owning a BMW performance SUV and something we just have to learn to live with.