A .58 a/r is incredibly small for a Supra. Granted it will spool-up unbelievably quick, perhaps quicker then stock twins would... but that turbo isn't going to be as effecient as if you had a bigger hotside. I would reccomend the .70 if you can get it. And in reality, the spool-up difference in time between the 2 would be very small... maybe 200-300rpms at most. Plus, the .70 would not be spinning as fast as the .58 would, but would produce more power, thus more effecient.
I know a guy with a SP63 who had the .57 housing and is switching to a .68(I beileve those are the trims) because the .57 was too small and could potentially cause compressor surge problems.
As for still being on stock fuel, whether you have the .58 or the .70, that won't really affect your fuel situation. Yes, you could build more power, but as long as you're at the same, SAFE boost levels, you'll be fine.
