Hmmmm, well, what I'm reading so far isn't really giving me much of a comparison between the twin & monoshock R100RSs. In fact, the July 1988 issue of "Cycle" likes the bike, but spends much of 7 pages bemoaning the weak charging system and Monolever rear suspension as compared to the GS' electrics and Paralever rear. They do mention the horsepower reduction and retuning for low-rpm torque, and also state that valves went from 42mm to 40 (exhaust), 44mm to 42 (intake), and the carbs were reduced from 40 mm down to 38mm. And, that the Monolever and a 1 gallon smaller fuel tank brought wet weight down 37 lbs, to 506 lbs ready to roll.
I did also find this at Whitehorse Press, http://www.whitehorsepress.com/product_info.php?products_id=3757 , and ordered a copy, so maybe that'll better answer my questions.
Inquiring minds and all... :-?