What's interesting about icepunk's Saito link, is that being on the Saito website, this fellow suggests going to a hardware store to buy the o-ring for the accessory stack.   It's true that the stack doesn't come with the necessary o-ring.

About the efficiency of the stack,  the new Alpha series of OS 4-strokes have added the stack and they talk it up big in their advertisements.

Thanks for all the good info!

Ernie