I've never heard of anyone running +1mm valves in a SX head. It is a more popular mod on the XC heads, but that's a completely different engine family. I don't know if it's because there's no room to go bigger in the head, or if the stock size valves are already big enough to flow the volume of air that the rest of the intake can deliver. Members on here have hit a strong 65hp with cam(s), good port work, piston, open intake, 43mm carb bore, exhaust, and ignition. Also, the factory Ti valves are produced by DelWest and are really high quality.