I'd say it's mostly "a", , they really help leading/trailing edges andhelp create space and separation.
There may be some of "b" going on as well but IMHO, it's not as much as people may think. That being said, different bypass caps of the same values do seem to sound different...
As Danny mentioned, the Sonicap G2's are a great buy and can really help the charge/discharge rates of a large bundle/valued cap. Give somea try and if you like what you hear and don't mind spending a few extra $$'s, try something different and see if you notice any differnce.