Turning on speedstep, turning on turbo and turning off turbo power limit, then setting the multiplier manually, seems to have cured my problems. Thanks for the (decidedly non-obvious) hint!

Currently running at 133x30 with headroom to spare. With speedstep off and turbo off, throttling ensued at 133x28 or 135x27 for me.

Now to try tuning my 2000-specced ram to perform at >1866.

Excellent to see such support. Thanks illuminatiASUS, unclewebb!