NEC ND-3500A and difference between 8x and 16x burning speeds

Hello all,
Though this is my first post, I've been a loyal reader of this site for quite some time. Reviews and this forum has been an invaluable source of information whether I've needed help buying the right equipment or choosing the right media. So already in advance I would like to thank all those hardcore senior members taking their time to answer the million questions popping up. Here is a few more. But first some background.

I bought my NEC-3500A over a year ago based on the good reviews and user comments. After setting it up properly (right firmware and media) I've been using it without any problems. My main use is to burn DVD-movies to my collection to be watched with a standalone Samsung DVD-player. So quality and compatability comes first and speed second. In the beginning I did some experimenting with non-brand media but very soon ended up with Verbatim media (4x and 8x, MCC002) which, atleast with this burner, has been a superb media. I have around 500 burned Verbatim discs with zero coasters and zero drops in movies so far.

Time moves on and so I decided to try Verbatim 16x media (MCC004). To get the most quality (writing strategies) of the drive I also decided to upgrade the drive's firmware from 2.18 to 2.1A. At first everything seemed fine. I did some 20 burns but then it hit me, a drop in the movie, and couple more. It was time to investigate what was wrong. That proved to be more difficult than I expected. I quickly realised that 3500A does not support disc quality tests. No problem, I had access to LG GSA-5610 external drive, I could use that. Doh, same thing, does not support quality tests. And here we are now with a bunch of questions for you:

1. With these two drives (NEC ND-3500A and LG GSA-5610), is there any way to verify the actual writing quality of burned discs? I've read about an unofficial firmware for 3500 which enables PIE/PIF testing with tools like KProbe and Nero DVDSpeed. I've also read that transfer rate test results somewhat indicate the quality of the burn. I've included the transfer rate test results from Nero CD/DVD-Speed for both drives (ran with the problematic dvd). They seem very nice in my eyes (if you don't mind the Finnish UI).

2. Is there any way to limit the maximum burn rate to 8x for all media? I'm using Nero 6 for burning and I know that I can set the burning speed in the Nero burn-dialog if I insert the empty disc before pressing Burn-button. But it would be very nice not to have to do this on every burn but instead have a default setting.

3. I've noticed that the 8x and 16x burning takes a lot longer than Nero has estimated. Does this mean that there is something limiting the burning speed or does the drive itself somehow regulate the burn speed or is Nero simply wrong with it's estimate? DMA is enabled and the data is coming from a 250GB Samsung SATA-drive, so that should not be an issue. Also the read-buffer is full all the time. But the drive spinning seems to vary. Could this cause worse writing quality? With 8x Nero estimates the burning time to be 7:05 when actual is 9:50 (disc size 4371MB).

4. When I insert an empty disc after Nero requested for it, everything seems go more smoothly; there are no delays in starting the burn and the disc is properly recognised after the burn. When I insert the disc before, to be able to set burn speed, there is sometimes a pause of 30 seconds before burning starts. Usually in the latter case the disc is not properly recognised after burn. Is this normal?