Ive been running one for the last 6 months, as far as i can judge it does nothing to improve performance ( which I already knew) and adds nothing negative or decreases performance.
It just makes a silly noise which is what I wanted as a throwback to my youth and my Renault 5 Turbo.
Not quite as loud as i would of liked but maybe a remap will increase that a bit.
If you want the noise fit it, if you don't like it take it back off, no harm done and easily reversible.