If you uncheck PoweRec in Nero, an info pops up and tells you the highest speed possible for the disc in kilobytes per second. I've asked them to include this as a disc info option for Plextor owners :).
It's been there a long time already In my Plextor Premium review I already saw it (scroll down to PoweRec):
PoweRec can also be disabled via the PlexTools software. A small benefit of this option is that when you disable PoweRec you can see at which speed your media would normally be written
Funny is though that the PX-712, even though it supports 24x writing on 32x CD-RW media, reports 0x as recommended write speed by powerrec (this is a firmware bug, not a Nero bug)