I have had both a CV64 and CV64 3D, with a scan doubler on an A4000 which I ran with both cards, and it worked.
The CV64 is definately faster.
OTOH both so much quicker than AGA, neither will disappoint.
The scan doubler was only 16 bit, but I didn't notice TBH. It eventually stopped working and I sold both the CV643D and the scan doubler.
I now have a CV64 and an Indivision. Once or twice I've run HighGraphics AGA just to see what the fuss is about, only to see that no, I'm not missing anything.
One of the cool things is that with OS 3.9 and BB 1-4, you can set up your Workbench using the PNG Kens Icons that are used in 0S 4. With a few speed tweaks it doesn't slow down at all, and you get to keep your full 2 MB of chip ram whilst running a 16 bit 1024x768, with all your backdrops, window patterns and super smooth Kens Icons. Stunning.
Oh BTW, the RTG software does make a difference: I've tried both CGX and P96, and I don't care what anyone says: for phase5 hardware, CGX is the superior system, if you set it up right and use CGX4 rev 6.