your 16/13 gives a 302% range. 16/12 will give you 327%. it is, however, all in the top end which, i know, doesn't meet your requested requirements. i was thinking of how to get most of the way there without spending a lot of cash.

anyway, i like charts. so i made a chart.

another option would be a 3 speed cassette. 19/15/12 would give you 28 - 108, a 389% range. whether you choose this kind of option or go with the schlumpf, i guess the thing to watch out for in the quest for a lower+higher range is the likelihood of gear redundancies across the spread.