1.No need for one,so why fit it.
2.The rough lumpy idle it'll give you,if the car idle's at all.
3.The very good possability that an air leak will appear and give rough running.Have seen loads of posts here with engine running problems that are casused by leaky dump valves.
4.The gay sound they make,every chaz in a turbo powered car fits them.

One "good" point
Fools outside pubs/chippy's and 15 yr old girls outside schools/youth clubs will think youre motor has 250 bhp!!!!

Regards Micky