Geez, no, I can’t explain the speed variations offhand. Are you using Nero CDSPeed Create Data Disc or burning actual files? If the latter, it could be a hard disc-defragging issue but could also be WOPC not liking that media (Ritek G05 can be variable to say the least). If you do a CDSpeed Create Data Disc and post the graph, it would at least rule one thing (WOPC) in or out.
As I understand it, the Plex firmware for the 740 is quite old versions of the original Philips firmware and I don’t think they’ve been updated in quite some time. You might want to try cross-flashing it to more recent BenQ 1640 firmware like BSLB or BSMB. At the very least, you’ll get new strategies for newer media, and possibly some refinement in existing strategies.
I think at least some of the 740 owners here have cross-flashed to BenQ and been very satisfied, for whatever that’s worth. Also, the BenQ firmware opens up a whole new world of Solidburn (similar to AutoStrategy), MSCE (strategy swapping), and CDSpeed scanning. Lots of “Read This First” stickies in the BenQ forum about cross-flashing and the new tools.
As for disagreements, that’s part of the fun here, no?