IIRC Nord Lead 3 even retains full polyphony after unison, but it's using some kind of oscillator trickery to achieve that.
Addixion: don't worry, a Jupiter 8 owner once asked whether his oscillators actually restarted if he hit the key. But it pays off to know a little bit about the internal structure of an analog synthesizer and how they are supposed to work.
When you have 8 voices, you have slots - the positions can be occupied or free. Once you release the key, the slot is free again (even though the voice itself may take some time because the envelope release was all the way up) - so once you hit another key, the oscillator at that place is tuned to its new frequency, the envelope starts at the initial attack position again, and you hear the new note.
With Unison, all slots are (usually) occupied simultaneously; all oscillators get the same pitch instructions. However, to get this "wide" effect, unison will also add corrections to all the oscillators - half of 'm are tuned down in a few cents w.r.t. the root note, half of 'm are tuned up.
With 8-voice polyphony, 8-voice unison results in 8/8 = 1 note effective polyphony.
With 8-voice polyphony, 4-voice unison results in 8/4 = 2 note effective polyphony.
And so on; of course, since you no longer have 8 voices playing simultaneously, it'll sound less overwhelming.
The best way to retain unison -and- polyphony is simply sampling each note you want to play, but once you change the patch, you have to resample everything again. SampleRobot - Clever Instrument Creation [ Sample Robot - Automatic Sampling ] SKYLIFE Software
can automate this process for you. The manual route is simply to be meticulous. It's best to sample a few notes and then check if it actually sounds good - because unison creates such huge sounds like this, it may sound less stellar if you occupy such a big part of the mix, and with too much detune, chords may sound off.