Fëanorian Quenya

According to the Shibboleth the more archaic dialect of Quenya used by Fëanor & the Vanyar distinguished between "s" and "th", while the newer variant used only "s" in pronunciation, but most of the Quenya words seems to exist in this newer version.

Is it reasonable to assume that "th/þ" would have been used in archaic/Fëanorean Quenya where the root or the Sindarin equivalent uses "th"? Or is there any other sign from which the older pronunciation can be infered?