Activist hedge funds such as those run by billionaires Carl Icahn, Bill Ackman and Daniel Loeb have more of an impact on a company than just forcing management to dance to their tune. They also shave more than $350,000 off a chief executive’s compensation within a year of taking a stake in his or her company. That’s according to new research from Warwick Business School at the U.K.’s University of Warwick, which compiled CEO compensation data from 244 listed U.S. companies and analyzed it over seven years.