I highly recommend getting a down cut bit for MDF. For fill thickness cuts it is not as much of an issue, but it will make a big difference on partial thickness cuts.
I cut way too fast on my XC with mixed results:
Cutting plywood with a downcut bit
But one thing I have noticed when cutting at higher feeds, depth of cut seems to really effect the load put on the router / quality. With MDF you should be able to push things pretty far but I would not do a cut depth more than 50% bit diameter, probably more around 40%. Even less if you have not done a stiffing mod.