Let me put it this way:
Nino Schurter rides what is probably one of the lightest bikes in the WC. He can get away with it because he's a wizard when things get rough.
His bike, by all accounts, is 7.8kg. World Cup courses have grown increasingly technical; there is a reason you see guys moving to larger wheels and slightly heavier bikes. The days of 15lb bikes on 1.8" tires are more or less over. Sure, there are a few, here and there, that still ride 560mm bars and bar ends, but for the most part, people are going with "heavier" bikes, wider bars, bigger tires. Those bikes are still under 20lbs, but not nearly as light as they used to be.
Same with local XC races. Now, I'm not sure where you ride. If your area is double track and easy, smooth singletrack, a super light bike with Maxxlites or Furious Freds may work for you. Where I live, and where I grew up riding (NY, and OR) you need some tread on the tires, some actual air volume, and wider bars to really haul ass. Plenty of people can produce a lot of power, and scream uphill. But races are increasingly won on the downhill and technical sections. Is that 500g you saved on tires going to help you, or hinder you? In most cases, particularly if you're all go and no show (road racer on an MTB), it will put you on your ass and out of contention.
I say this because I learned this the hard way. Once upon a time, in my early 20s, I was a Cat 1 road guy who started racing MTBs. I chopped my handlebars down and made everything as light as possible. I wasn't Geoff Kabush; I couldn't pull off Maxxlites. I didn't have the technical ability to do that. I'd blow people away on the climbs and lose that time on the descents. I went for a ride with a former Olympian MTB racer from OR, and he gave me some advice, on both bike set up and bike handling. I took it to heart, and applied to both my bike and my riding. A year later, I upgraded to Pro on the dirt.