ATM my monitor refresh rate on my Pi 4 is at 60Hz but the picture refresh rate is about 30Hz. Are interlaced modes still supported? Do they benefit picture quality? It strikes me they might well do.

You gave us no information about your monitor's capabilities. Maybe it's worth giving https://en.wikipedia.org/wiki/Interlaced_video a read. All my monitors are flat panel types, and are using 60Hz for their native resolutions.

Mine is a flatpanel also, a dumb kind of smart tv. Except for Netflix & youtube, getting online is such a pain I bought a Pi as a media box.

I was a tv techie back in the day, so interlaced modes are second nature. I just haven't compared them directly with uninterlaced modes performance wise. There's also limmitations on the A-72 core with 1 core, 1 thread. So you top out at 100% in top. With 1 core, 2 threads you top out at 200% per core. The trouble I'm getting from zoom seems to be mainly cpu related, when cores try to do 130% and find they can't. The Pi is still doing some software rasterisation. Drawing half the screen might be a lighter job.

I thought folks might have gone here and tried this, but they obviously haven't. So I'll try it myself and post some results.

Well, I spent a while experimenting with my RazPi and a while reading up from Debian & Raspberry Pi, and my results are in. Here's what I found out:

The Pis Do not handle interlaced modes. This has come up before because certain things have been linked to the pi built for tv monitors which naturally use interlaced modes. This is not good news, but as nobody is holding a torch to their posteriors they don't really care. With the Pi, video currently refreshes at about 30 HZ, Using an interlaced mode the apparent refresh is closer to 60 Hz. Of course only half the picture is refreshed every time, but the eye merges these little things and you don't notice it visually. More to the point, It halves the amount of data that has to be processed, which is significant. Hopefully the 6.2 kernels will do something about interlacing, but I'm not holding my breath.

On video overscan & edges, some strange things emerged

1. disable_overscan=1 totally disables overscan.
2. disable_overscan=0 with no other settings gives you a large top & bottom border, with a smaller side ones.
3. The comment in /boot/config.txt that this setting is only needed for ancient Analogue monitors is inaccurate. Many digital monitors need adjustment as well.
4. After adjustment, I am currently running tight on all edges with these settings:
   Code:

   disable_overscan=0
   overscan_top=-32 (adding some top back)
   overscan_bottom=-16 (adding some bottom back)
   # overscan_left=0  
   # overscan_right=0

On gpu memory, some are reporting improved video performance with increased memory, but they are few.It strikes me debian may have patched something in their kernel?

On GPU overclocking, there have been several surreptitious updates to what's allowed via the RPi firmware. If you aren't on the latest firmware, all bets are off, and you may get up to 750-800MHz. If you are on latest firmware, gpu overclocking is allowed to 550MHz only if you are on 4k mode. Otherwise, it's ignored. My firmware version (shortly after the usb boot was added) is allowing an overclock to 600MHz. [I finally noticed vcgencmd in /opt/vc/bin, but it's not in the path and it's man pages aren't in the manpath ]

EDIT: It now seems to me that 2 of the biggest drawbacks I overlooked in assessing the RazPi were

1. 1 core, 1 thread in the CPU cores, instead of 1 core 2 threads. I never really thought about it before, but it halves the cpu power, as with 2 threads, a cpu can go near 200%.

2. Lack of interlaced modes in a lacklustre video setup.This ensure that it stays a lacklustre video setup.

As a hardware guy used to assessing hardware from specs, that's a revelation.