This isn't really a "mod," but rather serves to slightly "deck" your cylinder, thereby increasing squish (and compression) and lowering the transfer and exhaust ports, which should boost both mid- and upper-range power. The difference between the stock .025 (inch) base gasket and the thinner Cometic gasket is probably only .010-.015, and on a stock piston/cylinder/head will not cause any problems so long as you run high-quality premium (93 or better) pump gas. You may want to keep an eye on your jetting after you do this, though.
On our highly-modded CR125 kart racing engines, we run a +/-.002" range for squish, and increasing squish by that much would cause heavy detonation and eventual grenading of the top end. Not probable on stock jugs. Hope that helps...