Yes an HDR at equal resolution will be equally sharp.

The 8K and 3K mentioned in context of the basketball sIBL set is the width of the images in question. The 8K by 4K image is much sharper than the 3K by 1.5K image.

I do not know for certain why sIBL builders think it's important to make the HDR smaller. I suspect it is because render engines that use HDR images for lighting (other than Poser) perform a more expensive diffuse convolution, and on a very large image this would be extremely slow.

Poser doesn't incur a speed penalty by using huge images, so for us it would be better if the HDR was deep and big. (Deep referring to bit depth being much more than 8 bits per pixel.)

With sIBL sets (as we find them, not as required) we are forced to choose the deep or the big - we seem rarely to be offered both in the same image.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)