[skupaemauto]

Officially its 2.7% of the country. But in my unscientific estimation in genetic reality, the real number would be at least 30% of the country. Whats your opinion on this.

The low official number is just people who self identified as two or more races.

So rough guessing...
About a third of the whites have some admixture, so ~24% of the total U.S. population. Around 60% or more of Blacks show admixture (probably more, going by memory), which would be around 7% of the U.S. population. Add the 15%+ Hispanic/Latino (which of course some are a single race, but for simplicity I am counting them all as mixed). The 2.7% who count themselves as mixed already.
Let's see 24+7+15+2.7= About half of the population is mixed to a certain degree or another, add in my lousy math skills and not taking time to carefully look at the numbers and I'd say that well over 50% are multiracial. What they self identify as is a whole other story.