IF you run a 6x9 that's more efficient then the factory, even though it has a higher impedance, should not notice much of a difference. Why don't you get yourself into a 4 channel amp and give the whole system a boost ?!!?!?
Current : 2013 Mini Countryman JCW
Past: '88 E30 M3, '69 BMW 2002 Ti, 2000 328i, Audi A6 TT, BMW X5 3.0i , Audi A4 1.8T