8x burning of 4x Verbatim DVD-R media is not necessarily problem free. It was problem free in my old drive, but the two times I tried it with my replacement drive, I finished up with discs unreadable in the ND-2500A beyond around 0.8GB (when the first transition, to 6x writing is made). See this thread. Note that this media writes perfectly (so far as I can tell - I don't have a Lite On DVD writer to run KProbe) at 4x in my new drive.
At some point over the next couple of days, I'm going to see what my results are like on Verbatim 8x DVD+R and 4x DVD+R media. I've got three discs of each for test purposes. I'll start with 8x DVD+R at 8x - if that's bad at 8x, then my new writer may have duff calibration data for 6x and 8x writing, and I'll look to RMAing it. It'd be nice to lay my hands on some supported 8x DVD-R media in case the problem is with DVD-R only - note that the media list only recommends 4x writing on Verbatim 4x DVD-R even though 8x is supported.
Verbatim 8x DVD-R media has only just come into stock at my usual retailer, blankdiscshop.co.uk - I'll get 3-5 discs on my next order. However, Verbatim 8x DVD-R is probably not in the current NEC firmware - the media list doesn't mention Verbatim 8x DVD-R - so it may take a firmware update for these 8x DVD-R discs to work properly (hopefully that's enough time for the price to fall a little!).
It's getting to the point that NEC really do need to get an official new firmware out for the ND-2500A - Ritek G05 8x DVD-R and Verbatim MCC02RG20 8x DVD-R are now available to buy in the UK and, I guess, other countries - and I don't think there's firmware support for either.