Well, that is the difference, it takes less powder in the 30-30 to get there but in theory more pressure, since we are burning a smaller of amount of powder to get the same velocity. This could play hell with trying to get an alloy to balance with the pressure, but the two great advantages of the 30-30 for cast are 1) the long case neck and 2) obtainable velocities in the 30-30 are just about the maximum practicle velcoities for a cast shooter anyway. There are a few who may dispute this, but for the average cast shooter 1800 fps is far more common than is 2300.
Be warned though, my Handi barrel has a bore diameter of about .305. The bore rifer of the Lee 180 at .300 fallsinto my bore and won't shoot worth a hoot. I would highly reccomend a bullet of full groove diameter if you are going to use a Handi (one of these is on my wish list)