Update. So after all of the tweaks, I think that going with a 6mm 4-flute carbide mill was the biggest single thing. I first rough a thru-slot with a 4mm mill to reduce the stress on the larger mill as well as allow chip clearance. Here you see just the roughing pass. 0.5mm cut depth per pass, and leaving 0.05mm for a full depth finishing pass. If you remember, I'm doing this with a gantry router. Works well enough!
http://i1262.photobucket.com/albums/...2/SlotFix1.jpg