Is it advisable to download the new firmware for the PX 716UF?
Your PX716UF is just a PX716A in Plextor's external case. Their specs online are the same mechanically speaking so the firmware revision applies to yours as well. The firmware link here http://www.plextor.com/english/support/downloads/firm_716.htm in fact includes your model.
I have a PX716a in an external USB2 / Firewire case, Plextools reports that its a PX716UF.
I guess the firmware is the same in this case.
(Same also for my PX708a, reports as a PX708UF)