The XF weighs about 500 lbs more than a Miata, and braking distances really can't be compared from one to another. The XF does have great brakes though, owing some to the Michelin Pilots and the wide tires.

Some guys are runnning 18" all around; i'm not sure 17" wheels would clear the brakes. Hopefully someone else who has done it will chime in.

Off-topic, i had an '01 Miata, FWIW (as my username suggests). awesome car, took it down to R Speed twice for their open house.