It just depends on what you are doing, but 20 seems light. If you have very large files with lots of layers and a panorama you could run 100-200 gigs of scratch disk space. Some do just fine on 40 while others blow through 120 gigs in a flash.
So is 20GB sufficent as a scratch disk, considering I've loads of RAM and Video RAM, had 52GB allocated and noticed I never used more than 1%
Uh...Vram has ZERO to do with scratch...the ONLY aspect of scratch is the image resolution (size) and ram allocation. You seem not to undersand the relationships?
You prolly need a substantial multiplier in scratch size to your allocated ram...of you are allocating 10-12 GBs of ram to Photoshop you should figure PS will need 5-10X for scratch (even if it may not be used, it will be allocated).
20 GB is small...unless you have a scratch disk 2, 3 and 4, specificied...you can have up to 4 different scratch disks spec'ed.
With 16 GB installed, I would suggest 100-200BGs as the 1st scratch disk size..but a lot of that determination is dictated by file size and average number of layers, which you've not bothered to mention...are you working on small files with a ton of layers or large files with only a few–it makes a big difference...
Cheers have allocated 100GB for scratch disk