Is an additional underlayment recommended with this product, I see one listed in the specs as a recommended underlayment but the installation instructions seem to recommend against using one.

The only time we recommend an additional underlayment to be used is when there is a present moisture issue in the installation space. You will need to conduct a moisture test first in order to determine if the area needs a 6mm poly vapor barrier based on the reading it shows, this is the only underlayment we recommend and only if necessary. Apart from that we do not recommend additional underlayment's as they could add too much bounce or constrict the flooring from properly expanding and contracting as the weather varies/changes.