I managed to get one for the 32cc bike I have - it was originally designed for a tanaka powered go cart, so it needed some adjustment but the difference was amazing.
The top end gains 5mph - but the real difference is in the mid range and torque.
On a particular hill (layer hill) locally the 32cc alone would be doing 22mph and bogging by the end of the steep section.
With the pipe fitted the terminal speed was 28mph and able to accelerate if pushed.
It probably wont do anything for economy - but at 189mpg @ WOT it doesnt do badly.
A more efficient way of doing a tuned pipe would be some sort of vacuum controlled exhaust valve - when the bike is on the flat it exhausts from the normal *production* exhaust - when accelerating or going up hill, the valve opens up the exhaust into the tuned pipe to give more power @ less economy. This is basically what sports cars with 'tuned exhaust' systems use, although 4-bangers use variable backpressure more than a tuned pipe type exhaust.