1) Read speed is limited to DVD hardware (read firmware) and has nothing to do w/ file format or OS. You'll get the desired speed (read 8-12x) w/ a 16x DVD-ROM.
The specs suggest that the LDW-400D will read at 8x. Incidentally, DMA is on.
2) DVD write speed is based solely on what the media can do, not what your writer can do. If you have a 4x DVD+R writer and put in a 2.4x DVD+R media, you'll get 2.4x write speed.
My concern is that even with no media in the drive, Nero reports that maximum burn speed for the drive is 2x.