Matt is obviously the expert here, but as my understanding goes, the chambers determine the "shape" of the power delivery ( in conjunction with the porting) and such things as how steep the cut off of power is.

The cans determine the total amount of power available, since to silence the noises for road use means stifling gas flow for the most part. Straight through race cans don't have much slowing effect, but the length and bore size do have a power effect I believe.

Saying this, chambers will obviously affect power since a bad shape will work against the power pulses and end up stuffing burnt exhaust gases back into the cylinder. The best shape will work with the power pulses and scavenge properly at the same revs the ports operate at. With power valves you get a better spread of power, and having these correctly adjusted will help too.

The shape of both cones of the expansion chamber is critical to the way the power is delivered. As is the length between them - hence equal length pipes on a V means one exhaust looks longer.

I think the colour is quite important too.

It's all very complicated really...
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