True, future firmware upgrades will make the drive faster. Originally the drive supported 2x DVD+R DL recording speeds and now it support 6x DVD+R DL recording speeds. DVD-R DL support at 6x is expected to be released this month.
The rebate you're talking about probably has nothing to do with the increase write speeds since firmware upgrades are always free. A rebate means that you can get some of your money back after purchase. Some drives also have a coupon for free DVD media.
The best media you can use on your Plextor drive is Verbatim, Taiyo Yuden and lot's more. The Post your Plextor DVD quality scans here thread has a lot of information.