15-17" is pretty ridiculously low;my Safari's granny is a touch under 20",and I've only used it a handful of times(and I guarantee I've got hills as bad as you do). 69-79" is also pretty low;I'd constantly spin that out going down my local hills.

You'll want to pay close attention to your gearing before dropping the dosh for an expensive upgrade like a 6spd conversion or Schlumpf. How bad is it getting up your hills? Might be a good idea,in terms of both time and money,to swap a 16t cog on the rear and see if that fixes things before doing major surgery. Also,how much have you been riding(daily,weekly)? When I first started out on my old commute with a hybrid,it killed me. Took me a good week and a half before I could get into work without having to get off the bike and push,and a good three weeks before I could do it coming home. A few years later,I got tagged for weekend duty,and over the course of two days had to make the round trip 5 times on a cross bike(36/28 'granny') and managed ok.

It would suck if you spent all that money on upgrades,only to find out in a year's time you don't really need them,and in fact hate spinning out on the downhills.