Or check out the Schwalbe Furious Freds. And then the Maxxis Maxlite 285s (as in 285 grams). Personally I like the RK 2.2 Supersonic because at 465 grams for a 54mm casing, it's got a lot of volume for its weight (note that the RaceKing is an entirely different tire from the SpeedKing). They hold up a good while for a "raceday tire," too. But my new race tire to try out is the Schalbe Rocket Ron 2.25" in the tubeless-ready flavor. They've got a reputation for wearing fast, and are very expensive + hard to find in stock, so I'm saving them for later.

As far as going fast in an XC scenario, personally I find that a long, relatively low cockpit is important for me. Try rowing a racing shell... do you sit upright, or do you lean forward and get your whole body into the pull? The "trail-rider" position imposed by short stems and riser bars doesn't work well for me, I need the classic John Tomac-style XC cockpit.