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Last year For Burrington I ran 38/16, and for Bank road 39/19, considering those are pretty much the extremes of hill climb courses I would imagine that a 6 speed setup with a 15, 16, 17, 19, 21, and 23 sprocket would probably work quite well for almost everything, with maybe the odd change of chainring?
you certainly don't need the range from 11-25 on a hillc limb bike anyway, not unless its a course with a downhill in it or something, and then I wouldn't ride it.
A friend of mine did similar for the national last year. For the vast majority of hill climbs I cant see anybody needing a full range 10 or 11 sprockets...