No problem, they are really the same as a L67 head with larger valves, and I have been preferring the use of those valves in the series 2 heads for longevity and easy adaptability.
Larger valves work fine in L67 heads. The later series 3 engines' valves are slightly larger than the older ones and will work just fine but the valve seats will need to be ground to match. There are other options out there for valves also if you are looking for larger ones.
Depending on the exhaust system on your car you will hear a little bit more exhaust note. An actual HP gain number is hard to say because there are an aweful lot of variables with mods plus the fact that the car will need to have a good tune to get the most out of them, but I would say an easy 15-30 hp. Any aftermarket camshaft that is larger than stock will definately take advantage of ported heads which will flow better at higher lift #s than stock castings.