You used the wrong boss if you lost speed and burning the belt. Might have wanted to check the differences in lengths for the bosses when you figured out something was wrong like burning belt.
The Polini has better top speed if thats what your looking for. The Malossi is a high quality good all around pulley tho. You absolutely have to make sure the bosses are the same length if one is supplied, the one for the Malossi is shorter than the zx50, cobra, or the super 8s.