I've sort of been staring at this thread and avoiding chiming in, but as somebody who's owned a MKS-80, a Matrix-6R and a Matrix-12, here's some thoughts:
1. I've never had a problem with the Matrix-12 envelope sluggishness. They are definitely digital envelopes, and aren't as fast as the envelopes on - say - a Minimoog D; nonetheless, they're still plenty good for me. I think this is one of those urban legend things that comes up with a synth where they didn't make many of them, and you get a lot of guff spread by people who never played them or who are just cranky that the LFO doesn't go into the audio range.
Things to watch out for if you get a Matrix-12 or Xpander: dying encoders, dying buttons, dying VFDs (occasionally), power supplies that should be rebuilt by a qualified tech at the earliest opportunity, and that one stupid DAC that occasionally dies and where there are no good replacements.
2. On that note, beware *any* CEM synth if you're worried about chips dying or otherwise fading into obsolescence. Frankly, this worries the h**l out of me, as both my big polysynths are full of Rare Unobtanium (the Chroma, especially; it's basically the only synth that uses the CEM3350 so if those die I'm in trouble.) I don't know what the practical alteratives are, other than maybe selling everything and buying something like an OB-X where the discrete parts outnumber the IC parts.
3. If you want envelopes in a polysynth with analog snap (i.e. the envelopes are provided by ICs), you're basically looking at any of the select few synths that use the CEM3310: the Prophet-5 (but *not* the T8), the various Obies mentioned earlier, the Memorymoog, and... uh, I think that's about it, but I don't know what synths do, and do not, use digital EGs from Japan. The Alesis Andromeda is also plenty snappy if you set the optimizer to "fast" mode.
4. The Matrix 6 is not the Matrix-12. You lose the bonkers multimode filter, and you lose the VCOs. I actually picked up my MKS-80 in an auction for a rack of gear, which also contained a Kawai K5M (which never worked) and *three* Matrix 6Rs. I never figured out what the old guy whose studio was having the estate sale used three Matrix 6Rs for. Anyhow, don't buy it assuming you're getting anything other than a Matrix 6R. The envelopes are still going to be sluggish.
I think it's a good trade for an Alpha Juno-1 or MKS-30 though.
4. If your concern is envelope speed, do you really need envelope speed *and* polyphony? If you just want snappy envelopes, go get a little Eurorack and shove some Cwejman EGs in it. The best case I can think of for needing polyphony and fast EGs is if you want to play snappy syncopated funk parts with both hands on the keyboard - in which case, yeah, nothing does it better than an OB8 or a Prophet 5, and you just need to decide which flavour of funk you want. But that's the main legitimate case I can think of.
5. Two options you may not have thought of: the Rhodes Chroma Polaris (based around the 3374/3372) and the Akai AX80 (more 3372, but based on DCOs.) The AX80 is fun and includes one of the weirdest, and yet strangely most comphrensive, user interfaces ever. It's a bit like flying the Death Star. The Chroma Polaris is a very solid yet rather underrated machine; it's also now possible to get new membrane switches for them. The Prophet '08 Rack is also worth considering; it's hardly a Vintage Synth, but it does at least come with support from a company that's still alive. It gets a bad rep around here, but IMO that's mainly because of a poor keyboard action and some of the worst presets known to man. (There's also the whole DCO versus VCO thing.)
"We have to remember that Moogs, ARPs and all of the first synths had been created by raging madmen." - Jean-Michael Jarre