|I've got cometic headgaskets and haven't noticed any problems.
I DID have some coolant coming out the exhaust after the first 40 miles or so on my motor, but when I went to retorque the heads (arp studs, supposed to anyway) found a few were pretty loose. After retorquing no problems.
I've heard good and bad things about cometic, but mine are doing great so far. Try the retorque first