Not much. The power of SDI was due to the size of the beams, not the necessity for overpowering the 'dissipation'...
If I remember correctly, the SDI LASERS were to be eight or twelve feet in diameter. Would you trust a 1 millimeter diameter beam to stop an incoming ICBM by punching pinholes in it? Probably not.
You're also not likely to get an eight or twelve foot diameter rod to lase at one Watt, given the required size of the flash source/chamber.