Really nice and IMHO the best proof that there is nothing wrong with your drive.
I have made bad burn after bad burn using Maxell DVD+R media and Verbatim DVD-R media when I burn at their rated 8x (or in Maxells' case I can burn at 12x but that comes out worse).
2nd attachment is with Verbatim dvd-r 8x media (media code MCC02RG20) at 8x. Looks "acceptable" until the end... nothing like the nice scans I've seen posted here though.
Given the more or less perfect result (PIE wise) of the 16x +R above I think we can savely rule out a mechanical problem of your drive. Verbatim 16x +R are known to be of very high quality. In my personal experience this ist not necessarily true for anything that has written Verbatim and 8x (be it -R or +R) on it. My results Verbatim +r 8x printable were worse than your result. Non-printable was similar or slightly better than your result.
3rd attachment is with the same media at 6x
4th attachment is with the same at 4x.
IMHO proof that this media just isnt' up to 8x. Generally speaking a raise at the end of the disc can be interpreted as "you should slow down the speed"
I have kept telling myself (usually with Maxell media) that I was putting crappy media through it, since the 16x burn with the verbatim 16x disc came out so well. After trying these Verbatim dvd-rs (in the supported media list), and getting bad burns I am not sure if there's something wrong my drive or if I've been unlucky again in getting crappy media. I've been using plextools and roxio to burn..
The big problem is that even the fact that the media is on the recommended list does not make for a guarantee for the quality of the media. You'll want to read the introductory remarks to the list on Plextors website:
I'll give you a personal experience: In my hunt for quality 16x media i found TDK +R und -R 16x in local stores (TDK MID). -R ist recommended by Plextor, while +R is only considered compatible. Below are the scans for +R burns (with Firmware 1.04) and -R (FW 1.06) at 16x. In the case of +R PoweRec slowed down to 12x before worse things happened. In the case of -R the same happened but the result was (IMHO) generally worse than with +R. As you can see a 'compatible' media beats a 'recommended' one.
Are there other tests I can do to check whether I should try to get another drive?
Try to get hold of other known good media and run some more tests. If you can, try to get your hands on some Taiyo Yuden disks. (Plextor branded disks are a sure source, see their website for how they look) If the results of for examplex +R 8x TY either at 8x or 12x is the same than your +R 16x Verbatim scan then you can be certain your drive is ok and its the media.