WITHOUT A SHADOW OF DOUBT BALANCE IT, if you lighten it, the balance will be out, most likely by a long way, depending on the extent of lightening, if slotted and NOT balanced it will shake itself to death, and tear your engine apart.
A basic lighten is around £45 or so, just basically skimming out the humungous rim of metal at the rear of the flywheel, mine went from 8.9KG to 6.6KG from taking a lot from here, I then further had it slotted and that went down to 5.8KG, but the extra work of slotting wasn't as cost effective costing £70 for 0.8KG off compared with £45 for 2.3KG off, done again I wouldn't bother with the slotting, I'd have just asked them to make me a new one from scratch as they(randals make them as well).
IF you are NOT going to have it balanced afterwards then DON'T lighten it.
tabetha