I'll tell you what I tell everyone who brings it up. It's a BAD idea to lighten a stock flywheel. The flywheel was designed for it's maximum strength in that size and form. You take part of it it away and you're tampering with a purposeful design.
1 - If they don't remove the exact same amount of material from all sides evenly, it will spin off-balance. Even if it's less than 1 MM, it could cause problems, especially at 7000+ RPM
2 - You're compromising the structural integrity of it by effectively making it thinner and/ or disturbing the balance of weight.
This is something I feel pretty strongly about, cuz I always hear people talking about how lightened flywheels aren't worth the money. If you're serious about your car, they ARE worth the money, because they're redesigned to be as strong or stronger than a stock flywheel, only lighter. Ok, I'm done ranting now...