Polyphony/efficiency of VA synths is always an interesting topic to me. As Timo pointed out, techniques like hypersaw on the Virus, supersaw on the JP8000, or "detune density" (seems oddly named among the three) on the Ultranova can be thought of as techniques / cheats to approach the same type of sound with less processor consumption (allowing more voices to be played at once), whereas I think unison is more straightforward in describing what it does (on synths I'm aware of, at least).
I remember researching this deeply once upon a time, and I believe consensus was that the biggest difference in something like a supersaw of 7 oscillators versus 7 real oscillators that were slightly detuned would be that the CPU/DSP is effectively removing some of the harmonics that would otherwise be emanating from individual oscillators (harmonics that the naked ear might not hear anyway), reducing CPU load and thus achieving a fat sound with less processing required, the thinking being that some timbres would effectively cancel each other out or muddy the waters anyway, so there should be an algorithm that can take the signal from a single osc, multiply and shift it somewhat, then remove that which would not really be heard (by most, under most conditions at least) anyway.
That said, I have no insight as to how the algorithms are actually implemented and I'm sure it varies between synths. However many synths have recognized or emulated the non-linear nature of the detune curve of the JP8000, which results in some interesting characteristics of the final output signal, so many of them strive to implement the same kind of signal (at times paying some level of respect to the JP8000 in how the signal reacts to modulation or knob tweaking, or perhaps putting their own spin on things).
At the end of it all, the nova (the only VA HW synth I have to compare at the moment) behaves much the same way as the virus. If you use all actual oscillators + unison it puts a high load on the DSP, which will requires it to steal notes. If you use alternative methods to fatten each osc, you can get way more simultaneous notes out of it. The end result might not be exactly the same sound, but it's also unlikely that you'd notice the difference in most mixes -- at least you can always use things like compression and EQ to squeeze out timbres you feel are missing.
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