Re: Expansion Chambers
Most of the power gains would be derived from tuning the expansion chamber to the headpipe. Expansion chambers are designed to have a "certain" amount of restriction so that all of the expelled fumes do not escape. What happens is inside the chamber the exhaust wave will "bounce" back up the headpipe and back into the cylinder thus creating a more dense and powerful combustion.
You may not know it but in the exhaust there is alot of unspent fuel. If your exhaust system is tuned properly this extra fuel will actually backfill the cylinder. If you are not properly tuned your backfilled fuel may actually enter the cylinder too soon which could possibly lean out the motor. Or it could backfill too late and inhibit proper exhaust flow.
The design and use of an exhaust chamber it quite and art and science. When you get it right you will notice the difference.
As for adding a long section of tailpipe after the expansion chamber. I am not sure if you will experience much in gain. Afterall the motors are pretty small. Most gains are developed with a proper expansion chamber setup.