That's a perfect graph.
You may be able to push that media to 6x or even 8x. I managed to write a couple of discs with that type of result at 8x in one drive. However, if I try it in the drive I have now, the write quality is awful once the drive switches to 6x writing - so bad that the ND-2500A itself won't read it. The discs were so bad that rather than being tempted to regard them as barely readable archive copies of the data (they will read back in my Lite On LTD-163 DVD-ROM with a struggle), I shredded them.
You can only experiment - try one disc with ideally at least 4GB if not more on it at 8x and try CD-DVD Speed on all the DVD drives you have access to. If the curve is not like the one you posted, it hints that your drive and that media really isn't happy at anything faster than 4x (at 8x, the first 800MB or so is written at 4x, to about 2GB is 6x, and the remainder 8x).
At 4x this media seems very solid - and at 89p for inkjet printable jewel cased media, I think it's an excellent buy. (I do have a printer - in my case a Canon i865). 8x writing only saves about 5 minutes over 4x anyway, because of the 'zone' nature of 8x writing on this drive.
(I've got a few 4x and 8x Verbatim DVD+R from blankdiscshop.co.uk for testing - in particular, I'm interested to see if the 8x DVD+R works OK, or whether my current drive is bad at 8x writing, full stop. A better comparison would be some supported 8x DVD-R media, but that'll have to wait).
SVP (blankdiscshop.co.uk) now have Verbatim 8x DVD-R, but the price is still very high and I'm pretty sure there's no proper support for it in current ND-2500A firmware.