Every single loader, every engine, has this standardized differently (including starting angle!).
This what hurt the most in those comparisons... Fstorm loader is indeed mirrored.. It seems like lupaz corrected it in the last render, not saying much about it.. Guys, checking the technical side is crucial in those kinds of comparisons, so as explaining the process... Everybody was taking a conclusion on renders with different lighting.... That sucks.
A lot of posts above were complete garbage.. comparing real photos to renders with completely different subjects, no controlled environment etc etc.. No need to expand more, this is useless. Let's not make this thread a complete junk pile.
Please lupaz, keep conducting your current test, it's interesting. But please make it fair and accurate. First thing would be to make them 1:1. So no missing materials/maps (if something do not work in an engine, should be changed by something that both supports in both scenes), same lighting (seems to be a little bit off still, based on the highlights upstairs), and a matching exposure. This would be a good starting point on which you can iterate.
Also, Fstorm as no caching solution, it's BF+BF. Not saying you have to change this one but this might explain some really fine details discrepancy some are talking about. Fstorm was remapping the glossiness range from [0.4;1.0] to [0.0-1.0], is it still the case? This could have a huge impact so keep that in mind.