I've found higher definitely helps for accurate audio capture. A good rule of thumb with audio is just use the rail voltage. So 15V aim for 15V/uS. This will put the errors way outside the audio band.
perhaps by 10x
You can hear slew induced distortion with something like a 2520, especially the older ones. But that's also part of its charm, sounds great for rock n roll.
I am not aware of that criticism, but I never messed with 2520s.
Slew rate has been well discussed. Back in the 70s there were a handful of attempts to invent new rate of change related audio bench tests (TIM, SID). AKA "transient intermodulation distortion" or "slew induced distortion". I preferred the old conventional distortion tests IMD (intermodulation distortion).
The industry standard SMPTE IMD test mixed a 60Hz signal with 7kHz. Perhaps back when that test was invented 7kHz was considered high frequency. 7kHz has not been considered difficult for several decades. I modified my old Heathkit SMPTE IMD analyzer to operate with 19kHz and 20kHz sine waves mixed 1:1, instead of 60Hz/7kHz. This test very effectively identified slew challenged circuitry, with a distortion product down at 1kHz in the middle of our sensitive hearing band.
To repeat myself by the 70s we had excellent off the shelf op amps with more than adequate slew rate. That said I killed some brain cells after beer o'clock some nights speculating about a dedicated test to isolate and measure slew rate of change linearity. My speculation is that different amplifier topologies will exhibit increasing slew rate nonlinearity before actual slew limiting. Pretty much analogous to how audio circuits begin to get increasingly more distorted before hard amplitude clipping.
This actually supports John12ax7's practice of providing more slew capability than needed, just not ten times
more slew rate than needed! In my judgement this whole exercise was made academic by the availability of much faster than needed for audio, off the shelf ICs, so I abandoned my quantification pursuit. In addition I couldn't easily explain to other engineers what the numbers meant and how to use them. Perhaps to show how crappy some old (slow) designs were. For new designs just use a modern part and don't worry about slew rate.
JR
PS: to answer the question I raised myself a perhaps useful slew rate headroom of 1.4x is likely adequate, while unnecessary with fast modern components.
PPS: If you think slew limiting is a useful audio effect for R&R it is possible to design a path with slew limiting intentionally, but I wouldn't melt any solder doing that.