the lathe itself weighs 200lb and the bench im building weighs about
another 100lb. is that enough to make it not dance with me while
turning big off center/not balanced pieces? or should i fill the two
pockets i made with sand to add another 50lb total?

A stationary mass is a great mitigator for rotating mass, but not the end all
and be all. Stiffness of the lathe, headstock, and shaft are also important. A
two ton lathe with a 1/2" shaft poking from a sheet metal headstock is not going
to spin a 30" bowl for very long. Bolting a machine to the floor adds mass and
stiffness, too.

Dan