is the B6 in the MX5 EFI?
And a MX5 with a BPT would absolutly hammer.... Let alone a BPD. Also, the shape and size of the engines are very similiar, as such you wont be f**king too much with the weighting of the car.
I would go the BPT personally, having seen it in action. My little 1.8T FWD astina used to destroy 2.5LT R33 GTST's all the time... (stock ones anyway, modified is a whole different game.) and the MX5 is lighter then the astina.