if you're lowering your tire pressure, you'll see more uneven wear (specifically rapid shoulder wear, if your alignment is correct, you'll see more wear on both inside and outside shoulders vs the center of the tread). Lowering your air pressure affects the foot print that the tire makes with the road which will affect handling and wear mostly negatively and allows the tire sidewall to flex more which may slightly help with ride comfort. In extreme low pressure conditions however, this flex is very bad for the sidewall when rotating and can lead to a tire blowout.
it would be very difficult for anyone to tell you how much less treadlife you'd get from lowering your air pressure. Tread life is based primarily on driving behaviour, so if you're gunning your Y everywhere, you'll get only a fraction of tread life compared to someone who drives normally.