Hi, 
 
I am currently considering purchasing a fairly inexpensive USB DAC such as a Focusrite Scarlet 2i2, to use in tandem with my Paradigm Shift A2s. Will the DAC truly make an impact worth $200? I am playing music primarily through my MacBook Pro. 

Answered a very similar question here:
 
http://www.head-fi.org/t/775301/do-i-need-a-new-dac

The Focusrite 2i2 for $150 is a really good value overall with the mic preamp, headphone output, and speaker output.
 
A DAC is really only needed if your source output sounds bad and most laptop headphone jacks don't really go well with powered speakers. You would need a true line out source to go with powered speakers so there is no hum and buzzes at top volumes. Try turning your laptop volume all the way up then without playing any music, dial up the volume knob of the speakers and see if there is any noise and that will show if you need a DAC or not.
 
The Focusrite is an alright DAC and sounds pretty neutral with no noise paired with powered speakers.