I think you should simply try out whether it works by replacing the jdk within the Windchill installation. In the past we also ran windchill with a newer jdk even if it was not "supported" by PTC which only means that theydid not fully test it.
Apart from that i can't see any features in Java 8 urgently nedded for Windchill development.ay
By the way all the java applets work with a jre 8 installed.