Sierra: thanks for the thoughts - the specs on the short cage 600 RD says 28T max so should be OK there, and you confirmed my thoughts on the chain wrap question......
So current plan is to try the 38 front ring with the stock cassette on the back - a 5 minute change that yields some immediately testable results for my roads.
Next, try pulling apart the cassette and fitting the 27 tooth low gear. I will probably, for the sake of the first test, just replace the 24T low gear with the 27 just to see if I really need the "granny" (sort of) low gear. If not, probably just leave things with 38/52 front....
If the granny turns out to be the bee's knees, will experiment with different combos of cogs I have in hand.....if there are shifting problems with the RD I'll likely try out the SunTour just to see (it is a wonderful RD), but probably this is as far as I'll go.....unless, I get obsessive about proper progression and then I'll look into John's suggestion of the 7-speed cluster (to get optimal ratios) and sourcing a long cage 600EX.....
Thanks everyone for the suggestions!