Private-equity firms have announced a record $944.4 billion worth of buyouts in the U.S. so far this year, 2.5 times the volume in the same period last year and more than double that of the previous peak in 2007, according to Dealogic. So far this year, there have been five $10 billion-plus deals in the U.S., equaling the total in all of 2007, though still below the high-water mark of nine in 2006.