Hi @SABristol, yes, as long as your power is under 200W, you can trip the QA450 as much as you like it should fully self protect no matter how long the FFTs are.
Are you saying that at 80W you experienced a burned up load resistor?
The link below looks at FFT size versus THD. Take a look at the video at the end of the post to see the fast testing–55 points in 6 seconds.