The FAA just put out a notice about the weight class defining the B757:
In short, this brings the US inline with the ICAO, in classifying heavy aircraft as greater than 300,000lbs MTOW.
So this means that all B757s, including those B752s modified to exceed the old 250,000lb MTOW, and the B753 are now all classified as large aircraft. The same separation standards still apply. So this will mean that you shouldn’t see any more heavy B757s flying around, let alone ATC asking ‘are you heavy today or not’ in their attempts to apply the right separation.