Well for what its worth the Yamaha document is correct on one point.
When observing the raw data from the player optical unit (eye pattern), discs recorded using the Plextor GigaRec system (0.8x 0.7x 0.6x) always provide a much cleaner well defined waveform. Also Focus and Tracking signals are smoother.
This could account for the improved sound in the car player, easier to track and less chance of read errors.
When monitoring the error correction circuitry, there are less total correctable errors, per hour, when using the GigaRec system than a standard write or even a pressed CD. So the electronics are processing a more accurate signal.
As to sound quality, only you, and your ears, can decide that.
One further point the VariRec + Recording strategy is definately worth experimenting with.
As a Studio/Mastering engineer I use a Premium 2 for pre mastering.