I run a 13oz on my red 500 and can't say enough about it. Like your buddy says, it starts easier, lugs like a tractor, and is just so much easier to ride even when riding with my daughters just putn around. No more feathering the clutch just dump it and idle away. No stalling. You can have two bikes, a ripper without the weight and a 4 stroke with it. Off and on in minutes.
Only time I noticed it costs any power is when drag racing and 4-5th gear wheelies, even then it not to noticeable.
If the weight is made with more steel further from the center of the flywheel, it will have more of an effect with less weight.
My trials ( not trail) bike actually has two weights, each VERY heavy. It will out lug any four stroke I have ever riden.
For something really cool, check out Jeff Wards flywheel weight (pics on on here somewhere). It uses a brass collar around the outside of the flywheel which does a couple of things. It gives you more flywheel effect with less weight and places the weight closer to the crank bearing for better bearing life. The collar is held in place by the magent weight screws.Way to cool! I'm definitely making one of these, or several of different weights. You could likely get a machine shop to make it for roughly what a new FWW will cost.