Will two spacers between the cassette and the hub effect the chain line?
You're thinking about this from the wrong side of the cassette. All cassettes end at roughly the same point or the lockring wouldn't engage correctly. The spacers are to make sure there is significant compression, but because the cassette is still in the same spot relative to the drive side of the wheel the chainline shouldn't change. 

That said you might want to pick up some different size spacers and see if you can change from 2mm (assuming two 1mm spacers) to 1.5mm. My guess is the amount you'd be able to move the cassette inward is very small, but might be significant. Most bike shops have a few spacer sizes on hand will sell them for $.5-$1 each IIRC.