I have drank very old puerh from an old zhuni (pre-communist China) and then from a gaiwan and I couldn't really tell the difference as I was thrilled by the tea itself. Probably a tea master or someone highly experienced and sensitive to subtle tea taste changes would be able to pick up the difference, but I am not, and honestly don't care. To me it would be more of a visual experience, old zhuni pots are very pleasing to the eye.
Modern zhu ni made by a reputable potter is achieved by mixing clays, no chemicals are added as far as I know, but there are potters who do add them though as they are allowed to do it without legally exceeding the established limits; of course there are some who ignore that but this practice is not limited to the copying of zhuni but of any kind of yixing teapot.
This whole Yixing pottery business is far too complicated, not even the Chinese now how things are messed up.
In the meantime have a read through the following article if you are interested in how this works:
http://www.terebess.hu/english/yixing1a.html
One last thing, zhu ni clay is not extinct per se, it is from the existing mines but new ones could be open and more zhu ni clay would be available.
Check this out as well:
http://www.teachat.com/viewtopic.php?f=36&t=15208