As a very rough guide, going from M10 to M12 with a 1000mm distance between fixed and simple bearings (typical setup) gives a critical speed increase from about 950 to 1200RPM (according to one online critical speed calculator). Given the increase in pitch as well, that's a useful increase in max speed. I don't know what a well-built MDF machine can take by way of cutting forces and hence max cutting speed, although having a decent "rapids" speed is a good thing to have even if you can't cut at that speed.

I seem to remember someone showing pictures of a damper assembly that claimed to increase max speed recently on the forum; it used some kind of fork that straddled the leadscrew to stop or reduce whip with a lever arrangement to lift it out of the way as the nut went by. Whether or not it would be easier to just fit a larger diameter leadscrew in the first place is moot, although reducing rotational inertia by reducing leadscrew diameter is a good thing in itself. Engineering is very much the art of compromise.