I performed the test on another macOS, 10.15.7 this time. I was running scsynth with native Jack support like on Linux:
sclang sine-test.scd 750: 75%
extra-load-jack-client 9000: 83%
With hyperthreading turned off:
sclang sine-test.scd 800: 80%
extra-load-jack-client 9000: 83%