Another thing... If I would consider a BiDir as some lead to what lighting/Brightness I should look for, then it seems that renderings with lower(2) lightsamples gets me very close to what BiDir renders... So I have no idea what's happening when using high lightsamples value with PT+PT or HDcache...
Now I'm fine with using 2 samples if I can cause its "20min vs 50min for 16samples in those renderings at 2000x1500" but there might be something wrong going on when you need to up the samples in certain cases...