Yes it absolutely must be balanced after lightening, even though it may not seem like it, if it started perfectly balanced when rotating, removing metal will affect it, as there are ever so slight differences in density of the metal in the flywheel.
Driving mine with the lightened/balanced flywheel it's just so different, you need to relearn pulling away using a few more revs, and feed the clutch in over a longer period, the difference between starting to engage and fully up on the pedal is so small, fells like about 1"!!
I have about 40 miles on mine now, and it is changing for the better becoming more progressive in nature, I would seriously consider a hydraulic clutch conversion at the same time, makes a world of difference.
I find with the lighter flywheel off boost pick up is SO much quicker, the smoothness is amazing now I'm mapped just on a running in map, I was doing 30mph 5th gear smooth as a smooth thing.
Just make sure whoever you use is experienced in flywheel lightening, as they can explode if taken too far/made too weak in certain areas, I have seen the results of a f/wheel exploding, chunks went through the bell housing straight through the trans tunnel and severed a leg, guy lost his lower leg, it's a hell of a lot of energy to go astray.
tabetha