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This (Shimergo) works like a charm and is a perfect setup for touring, i run this on two bikes. Durability of 8-spd drivetrain plus modern shifting. You can even combine Shimano 8-spd MTB stuff with the ergopowers. This also works with a triple chainset.
There will be probably 1-2 cogs, where shifting isn't 100% accurate and spot on, but through finetuning the cabletension you can determine where this is and put the range to a not so often used area in the cassette.
No problem. Just set the limiter screws of your reasonably old shimano mech at the ends of your 8 speed shimano or sram cassette, clamp shifter cable, take up the slack with the barrel adjuster and shift away. I've had the same setup for years, there's loads to read if you google 'shimergo', i.e.
http://www.cyclinguk.org/cyclists-library/components/transmission-gears/derailleur-gears/shimergo
http://www.cxmagazine.com/shimano-campagnolo-ergopower-compatibility