I don't road race yet, but in XC and cyclocross I actualy stand less than when just riding around. I stil do, but I can feel the inefficiency more when I've spent 45 minutes in zone 4 or whatever. I atribute this to the fact than when riding by myself when I'm sitting I'm at a fairly relaxed energy output. When racing, I'm at a high power output even when sitting.
I've noticed the same phenomenon on relatvively long road rides.
Not sure if that helps, sorry.