As @YoMan noted, please be careful and keep potential toxicity in mind — I’m really sensitive to plastic fumes (give me a nasty headache) and even cutting acrylic on my Nomad at the recommended speeds and getting good chips get a headache (and I’m rather dreading my next project, but I need to cut some nicer hex bit holders).
We do have official numbers for styrene in our charts: http://docs.carbide3d.com/support/supportfiles/S3_feeds_250.jpg — is that number not working? What is your source for it? Are you sure it’s not some variant (e.g., like to cast vs. extruded acrylic) which makes cutting more difficult?
I’m afraid there’s not much in the wiki, just a metric version of the official numbers, and some untested numbers and a note on the difficulties of it: https://www.shapeoko.com/wiki/index.php/Materials#Extruded_Polystyrene
and put pretty much the one idea I could think of into that discussion — single flute O-endmill intended for plastics — there was one post which noted success and had some hard numbers, but they may not be applicable to your situation: https://www.reddit.com/r/hobbycnc/comments/3ty0tq/issues_with_milling_extruded_polystyrene/cz1ie1o/
Might work if you have a Makita and dial it down to 1 — if you’re using a Dewalt, do you have the adapter ring? If so, willing to try picking one up along w/ an endmill suited for plastics? List at: https://www.shapeoko.com/wiki/index.php/Endmills#Plastics