Hi SS,
There is a recent Instruction that states that a full 120 seconds separation must be applied between an A340 and a following Heavy on the same route. Having said that, I am personally very mindful of the problem and sometimes use the goundspeed function of the ATM to check the A340 speed. If it is excessively slow I apply another minute separation for good measure. It happened last night actually between a VIR A340 and a SAA B744 on MID departures. VIR was showing 165kts and passing about 1800ft 3 nm South of the airport, therefore an extra minute was applied. But then I am an experienced radar controller. Maybe it is part of the legacy of having ADC only controllers that they do not so easily appreciate "catch up" situations