can you see the difference between the various 422?

Proxy? Yes. The others, not really. It takes a discerning eye, a really good display, and certain types of action, shutter speed, contrast, chrominance, or other pixel content show the differences more than others.

There's a website out there someplace devoted to analyzing and comparing ProRes flavors. Can't be too hard to find.

I agree with David: Proxy yes, the others no*. I frequently use LT as an intermediate export before transcoding to H.264 delivery. There's an asterisk on "no" because I know there is a difference and certain types of media will require the higher ProRes codecs to preserve the images as best as possible.

All ProRes codecs are "lossy" BUT they qualify as *virtually lossless*. https://en.wikipedia.org/wiki/Apple_ProRes

The features of ProRes make it a better format than Uncompressed formats (IMO; better subsampling, etc.).

What kinds of media make a difference? Some gradients (banding). High motion (blocking). If there's anything in your media that just doesn't look quite right: increase the bandwidth. In ProRes, that means going up the quality scale (LT > 422 > 422HQ >4444 etc.) It also usually means you have to transcode at a higher bandwidth as well when going to your delivery format (h.264 typically).

PS - Proxy media in FCPX is NOT ProRes 422 Proxy, it is ProRes 422 downsized to 1/2 resolution (e.g. 1080 is converted to 960x540).

It depends on the media and the content. In 4K or higher, yes I think you can see the difference between 422 and 4444.

I think proxy, in addition to scaling, uses the proxy codec. The downsizing is a function of the format. For some media like RED FCP doesn't do this and doesn't use this format.