I've been racing my SS MTB for 3 yrs (Vet Expert XC). I'm now 1 race away from finishing my second season of 'cross on my SS....with a strong finish this weekend, I can claim 3rd overall in Vet A. I find that SS really helps in 'cross.
First, conditions are usually terrible (mud, snow, ice, etc.) and you can hear drivetrains suffering. With only 1 ring in front and 1 cog in the back and no derailleurs, shifters, etc. to worry about, the drivetrain just keeps spinning along with zero hesistation.
Second, yes, there is a slight advantage due to the light-weight of the bike. With no derailleurs, shifters, cassette, etc., the bike will be lighter....comparing similar set-ups, that is. This definitely helps as the race wears-on and the barracades seem to become taller.
Third, momentum is everything! I find a lot of racers/riders are too busy shifting or mis-shifting and loose a lot of natural momentum. SS'ing has taught me to savor every ounce of mo' and to use it wisely.
Fourth, just hammer! No need to worry about the optimum gear. You'll also learn to be much more agressive in passing, etc. since you have to conserve all of your momentum and to take advantage of opportunities gearies usually give you.
Fifth, for most 'cross races, you are going to be forced to dismount just prior to a big hill where you may not have the right gear. So, everyone is off the bike anyway.
About the only negative to SS'ing (in 'cross, MTB or road) are the flats.
BTW, I'm using 42x18 for 'cross. For MTB, I use 36x18. For road, I'll use 48x16 for fixed-gear riding and 48x17 for freewheeling.
To be honest, I've become a much stronger racer in all disciplines (MTB, 'cross and road) since I became 100% SS.
Try it, you've got nothing to loose (with the exception of all that shifting junk) and everything to gain.