It's not the porcelain, but the glaze on the porcelain that matters. And there are different glazes. The extent to which the difference matter in terms of taste, i don't know.
I have two pairs of the white ROC cups with blue rim: on one of them the glaze is much smoother and feels nicer then on another (maybe due to different firing temperature or different composition, i don't know). It may not make difference in a blind testing, but it definitely makes a difference in handling them.
This is a great point - these glazes usually don't interact with the liquid much (hagi is a different story)
, but the feeling of the glaze texture has an impact on the tea experience. The feeling of a smoother cup also matters when it comes in contact with the lips when drinking, and it can affect the perceived texture of the tea.
I am just not very convinced that this depends on how old the cup is - as you say your two ROC cups behave in different ways in this respect. The standard cheap porcelain cups usually don't feel very smooth, I have some ROC cups that feel a bit smoother, but then among my cups those that give me the smoothest feeling are made by Xu Dejia, just a couple of years ago.
I am not opposed to old cups in general - I appreciate old teaware and its historical and aesthetic value (there's a beauty just to thinking of how different the world was when these things were made, and who might have used them). I just don't think that in this particular case the linked set was worth its cost.
What is different about Hagi glaze, i'm curious.
Often hagi can be quite breathable and porous, sometimes it even has patches of naked clay, like this: http://www.artisticnippon.com/product/h ... mi068.html
so it can affect the tea a bit more.