A lightweight wheel would be about 10-15lbs for a 15" wheel and a 15" tire would be around 19-20lbs. So you're looking at the least 30lbs. per corner.

Si's aren't the lightest wheels out there, but at least they aren't the heaviest.

I'd get the 205/50/15 tires first and see how they sit. If you still see a gap, then lower it to the height that you want. But don't lower it more then get different tires.