In the U.S. the regs were rewritten to include the 757 as a heavy. The old limit was 300,000 lbs, and its now down to 225,000(i think). And the whole reason for this was for wake seperation!
At a previous company, we had a strange DC-8 that had a gross weight of 293,000 pounds. Some guys (nuts) insisted on telling the controllers that they were not a "heavy". So, they take off at 293,000 lbs, with 1 minute seperation,and the proper DC-8 behind them took off at 250,000 and with three minute seperation. see the picture here. Some of these guys couldn't see the forest through the trees, as they were so focussed on the details and not safety or reason behind the rule. Wake turbulence is extremely violent and should not be screwed with.