I think the efficiency for that price is very good.
I've bought this exhaust on my Brixton Felsberg 125. I went from 38mm to 44,5mm. And so far so good. The sound is like a beatifull bubbling thunder :D. And when you unleash this thunder, people think that I ride 600cc. Also the performance of the bike changed a little bit. The bike has a faster acceleration, thanks to the bigger pipe, and I think it's little bit faster. Anyway I think this exhaust is a very very good choice. Also guys from Café racer webshop were nice and kind. I did the order from abroad and the delivery was smooth and without problems. So, thank you for this guys. And if you've got here, go and buy something from these guys, they are awsome.