Right, Ground Zero (which I assume is a regular site of yours) limits DMR semi only AEGs to 425FPS. You'll need about an M130 spring to get you near that, but you'll need a whole host of other stuff to get it accurate.
Have a look at this: http://www.airsoftsniperforum.com/43-longrange-aegs/892-holy-grail-building-your-dmr-aeg.html
and then decide if the minimal extra range is worth the loss of full auto and accepting an MED restriction.