but isn't the difference with the 744 that it was provided with far more powerful high-bypass engines than the Classics were?
It depends, there were a wide range of engines available on the 747-200 - everything from 47k to 54.5k ratings. Baseline engines for the 747-400 were about 56k, but it was also higher MTOW than the -200.
I've been on a 747-8 flight test at 43k, Mach 0.78 (VMO Mach 0.9) so there was a considerable speed margin. However we also quite light, roughly half MTOW
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BTW, the -8 is only certified to 43.1k - while the airplane is fully capable of going higher, the FARs have changed with regard to depressurization. As a result, I doubt you'll ever see another Part 25 airplane certified to above 43.1k
