That's not really a good air vs water comparison. You'd have gotten similar results if you swapped for a better tower cooler as well. The stock coolers are basically built to cover base clock and a bit of turbo.

Yeah with a CoolerMaster Hyper 212 and a single fan I get idle of 40-45 and a high of 76 and I tend to run with fans set to be mostly silent. With a bit of a noisier experience I seem to recall idle was under 40.

That is all OK, but remember that Zen 2 really tries hard to hit 90C. Especially if you go into Ryzen Master and turn on Automatic Overclocking (very conservative overclocking by enthusiast standards), and your motherboard doesn't have power delivery limitations, the CPU's main goal is to try to hit 90C under load. It will automatically adjust frequency and core voltage up until it hits a thermal limit; by taking more heat away from the CPU with better cooling, you end up with better performance because the CPU will try to automatically give you the best performance that your thermal solution can deliver. AMD didn't document any of this very well (lots of confusion between Precision Boost 2 and Precision Boost Overdrive), but the technology is very impressive.

I guess what I'm saying is that you might be interested in exploring some of these options in Ryzen Master. You have some thermal headroom, so if your motherboard has good power delivery, you can max out the power limits and boost clock limits, and probably squeeze out a little extra performance. Power (and therefore heat) is going to scale quadratically with core voltage, and frequency is going to scale less than linearly with voltage, so you might not see much gain (and might prefer not spinning your fans up to full speed under load). But, it's an option that's available if that sort of thing interests you.

I got about 6% more real-world speed out of my Threadripper 3970X by switching from OC off to Automatic Overclocking; with air cooling. AMD's technology here is pretty impressive.

I didn't see this reply until just now.

I had no idea what Ryzen Master was until you mentioned it - seems it's some AMD utility for Windows.

I'm running a stock hardware configuration, no overclocking, and running Pop! OS 20.04.