I've got an aluminum flywheel (Vintage Performance Developments -- the street model, not the race one with the slots), so I don't know how much is safe to take off the stock flywheel. 4 lbs off the outer rim is a lot more effective than 4 lbs shaved off the whole thing.

Pro: The engine revs quicker. This is noticeable on acceleration in the lower gears particularly. A true "free" performance boost that doesn't show up in HP or torque figures, and doesn't reduce economy.

Con: On mine, it's light enough that it's impossible to hold steady speed in first, and dang hard in second -- without enough flywheel weight to smooth things out, there's an oscillation that builds up until the car's bucking. Not good for parking lots or being stuck behind a semi-truck downtown. Also, heel-and-toe downshifts need to be done WAY more precisely than before.