Additional info: from Corona 4 Beta, a much better bump map than the previous versions was introduced, as far as I see. Now, when I open old scenes and re-render, I see even the pores in wood surfaces even in total low res maps.
Such I always wanted before...
In my oppinion the new bump map became so good that I only rarely have to switch on displacement. Saves plenty of render time.
Well done Corona team !