Here's the deal as I see it: the newer pxdrv.dll files (as used with current Sonic/Roxio applications) don't appear to add new recorder support for RecordNow Max/DX any more.
Therefor, if you want selectable burn speeds with your new drive, you have to manually edit a pxdrv.dll with a Hex Editor (e.g. Tiny Hexer), which is as easy as substituting/renaming one of the existing models listed with the exact name/drive string of your burner.
You can start with the pxdrv file and installation instructions available here . Note also that PxEngine updates available from Sonic and newer than 507 will break your installation.