Use of samples per pixel per iteration ( useful for multi-GPU )?

I have made renderings at irendering with 4 x 3090.

About the speed / efficiency with this setting samples per pixel per iteration, here are some test:

best I think. 17 secs per frame

1000 samples per pixel per iteration
50 Path Trace samples per pixel
20 motion blur subframes

difference is almost unseen. 17 secs per frame

500
50
20

difference is almost unseen. 17 secs per frame

50
50
20

Now this is slower, 32 secs to render. So we do not go there.
5
50
20

So how should I use this? Is there any reason to raise this even higher like 3000?
Or should just use higher sample value (the one that I now have 50 all the time) ?

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