OK, concrete examples:
Quadra 650, System 7.6.1 for AppleShare IP access to X.4 so as to transfer images, 128MB RAM, used primarily as a scanning station with a Color OneScanner 1200/30, software used mainly Photoshop 4 and Acrobat 3. I have a Storm 2xDSP Nubus card in this machine (no video output/port from this card, though), so that complicates the question somewhat. VRam maxxed, of course; also has a PDS Quad 040 accelerator @ 40MHz with cache. Monitor a 1024x768 Philips LCD. Very fast, highly upgraded Quadra, mint, still with one spare Nubus slot. Would it benefit from one of the cards below? Maybe.
Quadra 840av, with a SpigotPower AV card specific to the AV (again, no video output from this card, which is used for digitizing video rather than displaying it), 96 or 128MB RAM, I forget. Running 8.1 for HFS+ support. VRam maxxed. General software packages installed include Adobe products as above (though I really just use the 650, which is faster in the configuration given), but this one also has Premiere and Deck and such, since I intended it on set-up to function as an actual AV machine. Would it benefit from one of the cards below? Probably not.
Quadra 950. I have two of these; one is an A/UX box, where Nubus video card support is more or less non-existent. The second one is not used as more than a shelf support currently, but I would like to bring it into some kind of service. 64MB+ RAM (I have some 16MB simms, plus a collection of 4MBs, so it can be brought up to maybe 96MB or some such), and VRam is already maxxed. I would probably go here with an earlier System 7 installation (7.1 or 7.5, which I happen to like), and so the question: do I equip with Nubus video or leave it as it is?
At the high end of the Nubus pile, I have available a Radius PrecisionColor 24x, a Radius Thunder IV 1360, a RasterOps 8/24 XLi, an Apple 8-24GC and a Futura II. All are unused.
I thought it interesting that CelGen, who like me has tons of this stuff, thought subjectively that there is not much between the onboard video of a Quadra and a good Nubus card. That's been my sense, but it occurred to me on thinking about doing something with the 950 yesterday that I might be completely wrong, and that is why I am asking.
Is the truth that what really matters with this gear is the Control Panel, and what it makes the card capable of doing? So, e.g., the card might not be faster, or much faster, than the onboard video of a Quadra, but can nevertheless do more interesting things? Maybe the advantage is really the pan & zoom/ virtual desktops/ G-worlds etc. that become possible rather than just general Quickdraw acceleration, so that this would make it worth doing the upgrade. After all, it is the software that makes the hardware run, and it is software that we actually use.
I would probably make the 950 a Claris software haven. So QuickDraw likely matters. As for thousands versus millions of colours, I can't tell the difference on a 68k machine, though it does become obvious enough even to me in OSX.
I suppose I could always just stick all five of the cards available in the 950 and have a shoot-out, but something tells me that the left hand control panel would not know what the right hand control panel was doing.