Warren Buffett and Charlie Munger share their M&A philosophy, They explain how Berkshire's size is becoming a drag on performance and its ability to compound returns. They warn why Berkshire may underperform the S&P 500. They address Berkshire's performance in Nevada. Buffett reveals that of investment managers Todd Combs and Ted Weschler, "one's modestly ahead" of the S&P 500, and both are beating Buffett. And they discuss American Express' competition.