I'm not aware of the diffuser on the Evo. Are you confusing a diffuser with the "vortex generators" on the trailing edge of the roof of the Evo?
I know there's a guy around who has a front air dam with a front splitter he made with Home Depot stuff that doesn't look half bad. He might pop up. Last I saw of him was on the old form a long time ago I think.
Front air dam + splitter seems like a good idea. Might try an under tray for the front too. Rear diffuser is an okay idea, but might ruin the looks more than you think if done too aggressively. I'd aim for subtle.