Check out rq3's extruder threads in the tech-talk section. He's gotten as low as 42 grams including the motor.
As for the X motor, it can at least be a shorter length NEMA17 than the Y axis. NEMA14 may work, but my guess is that you'd be better off enlarging the Y motor instead. The smaller motor would allow higher Y acceleration without print quality issues due to frame rigidity/vibration, but whether that will result in higher usable print speed depends on whether the NEMA14 can accelerate the printhead at the same rate without missing steps. Ideally you'd want to rig it up with an encoder or other measurement method so you can experimentally find the maximum acceleration rate you can safely use.
If you make the X motor closed loop, then you can push it closer to the limit. You still don't want to be missing steps on a regular basis because it will show up in the print as delayed motion, but with open loop you need a wide safety margin to be sure that the whole print isn't ruined by one missed step.
But really it seems pointless to go to such lengths when you could switch to CoreXY or crossed rail and eliminate the moving motor weight entirely.