234m americans over 18 [2010 census], *20k ea =$4.7t
GDP $15t (assuming this doesnt dip with the bottom half of the workforce checking out or thereabouts calculating their distance from $20k and adjusting workload, or if implemented on a sliding scale determining their particular work/pay ideal balance)
So I guess it could work. Personally I'm suspecting the '47%'ers or whatever hit-above-their-weight a bit, and dropping them out of the economy and replacing them with a $20k handout would drop our GDP a bit from $15t, making it a approximation equation that eventually stabalizes at a new GDP(t) probably arround $12t or so. Now at this point the economy has a much harder time providing that transfer payment while other social services/needs are met.
Or perhaps it goes the other way, and these people are served by loansharks that offer great financing rates for their super-stable income levels. Perhaps they become an even BETTER consumer and this ticks the GDP up a bit more than expected and everybody wins.