I have run all the pipes discussed. I liked the Blitz Imperial Pipe. It's great quality, however the pipe is designed for a stock 50cc cylinder and increases low end/mid range, however, it was a bear getting to 40mph. It looks like the pipe is too long (too much length of piping). Also, if you noticed the expansion chamber is symmetrical. I usually like a more agressive convergence cone compared to the divergence cone.
The bi-turbo is so-so, it works good for mid range/top end, but the lowend sucks. I got my bike to go 44ish (gps) with the bi turbo and uni pod.
The technigas next R is a very good all-around pipe, but you hit a wall at about 42mph. I know a couple people that have shortened this exhaust and are now getting out to 45mph.
My favorite, that I'm running now is the A35 Bullet Pipe I had to cut/weld the pipe to the get the exhaust flange angle correct for the A55. The header pipe is much larger on the A35 pipe vs the A55 pipe. Another note, if you compare the bullet pipe silencer to the bi-turbo silencer, you will notice that at the end of the silencer the exhaust comes out of a tiny hole and that what looks like the exhaust tip is actually just cosmetic. The A35 bullet gave me more mid range/top end compared to the bi-turbo. I was doing 45mph on my bike with just a unipod and the bullet.
Another awesome choice would be the techno estoril. Once again, it is designed for the A35, so you will need to cut/weld for the A55 exhaust angle. Broader power range than the technigas. Will get you out to about 44-45mph, with sick low end/mid range.
BTW, these exhaust comments/advice was done on stock 50cc A55 engines with unipod
Hope this helps.