You can find my experiences with the Premium 2 here: http://club.cdfreaks.com/showthread.php?t=193482 . (You'll find some explanations of how to interpret what you see there here: http://www.cdfreaks.com/reviews/Plextor-Premium/Q-Check.html) . It is indeed a very fine CD-writer but if your current drive and playback solution is working fine there is little need to change anything.
GigaRec and AMQR can enhance the readability of a disc. This can have a positive effect on longevity (a already decaying GR/AMQR disc may still be readable over a regular similarly affected disc). Similarly if your playback equipment is picky on disc quality it may help avoiding interpolation of the read data, pops and clicks and thus indeed 'sound better'.
If you can't hear a difference then it is probably because your LiteOn and playback device are getting along quite well
You don't need extra software. The included software (Plextools Professional (LE)) allows the control over all the extra functions and any third party software can be used to do the mastering/writing etc. The commercial XL version of Plextools allows for some extras, which are (IMHO) more for convenience than necessity.
As for special mastering software for transferring vinyl i don't have much experience and cannot comment.
The Premium 2 with Plextools also allows to check the quality of the burned discs. AFAIk the LiteON drives offer such functionality too. But I don't know about your drive.
To sum it up: If you're really interested in producing quality CDs the Premium 2 is worth it's money. Overall i'm under the impression that DVD-writers give worse quality on CDs than pure CD-writers. Since CD-writers are getting more and more rare it might be worth to get a known good one as a spare. If this makes sense for you is up to your needs.