Another solution that I haven't heard might be for you to use a demand
heater on the cold water supply to the shower, not just one to the existing
gas water heater... That way, the cold water that is being mixed at the
shower head is not quite so cold and it wouldn't need as much hot water to
get it to a comfortable temperature... At the very least, a demand heater on
both the supply to the hot water heater and one on the cold water supply to
the shower combined with increasing the temperature setting on the existing
hot water heater should work...