I have just replaced the Sito Plus racing exhaust on my Malossi 166 with a SIP Road XL racing exhaust, and this is my observations so far: The SIP XL is much quieter and has a very nice sound to it, sounds more like a Vespa should sound. The Sito Plus has quite a tinny sound to it and is very noisy. But the Sito Plus gives a better performance and top speed. With the Sito Plus I got up to 105 km/h (65 mph), but with the SIP XL I only got 96 km/h (60 mph), a difference of nearly 10 km/h. (The exhaust outlet of the SITO is also much bigger in diameter than the SIP; and I think it shows in the performance).
However, the SIP XL sounds good and gives a smooth acceleration, thus I will stick with the SIP. And 60 mph is more than enough!!