The EML PX4 mesh tube is upwards compatible to the 15 Watt Version of the historic PX4 by OSRAM and MARCONI. The EML PX4 has higher heater current, and also higher maximum power ratings.
The PX4 is a well balanced design by Osram and Marconi. It works best at the recommended operating points. The higher dissipation as possible for the EML is only for extra lifetime and more safety.