Here is the conversation I had with Aram replying to my support ticket, in case it helps:
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> Aram Avetisyan (Chaos)
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> I understand what you are addressing, but keep in mind that the filtering has been improved for environment maps, so consider the results with Corona 10 as "superior" now, even though they differ from Corona 9.
What do you mean by "superior"? All I can see is less light being cast by the HDRi and blurrier shadows which to me looks like the opposite of a superior result.
> The "fix" for this already implemented - the better filtering - which comes with a drawback of possible differences between Corona 9 and Corona 10.
What was wrong with Corona 9 filtering? I still don't understand, sorry. If the goal was to speed up the rendering, I understand that, but not at the expense of a regression in shadow rendering and lighting intensity (both of which being linked obviously).
> And there is a workaround, I would say rather easy one, the string option, to get consistent with Corona 9 results using Corona 10.
Yes there is a workaroud, but it's far from user friendly.
> I have just in case asked clarifying questions for devs to see how exactly the resolution of a map affects the filtering, and I will update you once there is a response.
Great, thanks Aram.