Phil,
I too had issues burning to DVD+R and DVD-R. After several attempts at creating non-bootable copies, I went to a different machine with a different brand burner and the ISO burned and booted first try. Strange thing is the first machine burned and even verified the burns using ImgBurn. Hope you get things worked out.
Speed of burning, and using, or not using, RW media, is pretty much false information. If you have a burner that is working properly, AND you have good media (any type), you should be able to burn a good copy, at full speed, every time. Unfortunately, burners tend to wear out, with use (especially if you use one of those head cleaning things), and media quality has really gone downhill over the past few years. Then there is the problem of dust balls collecting inside the burner. They float around, and get in the road every once in a while. If you can burn, and verify the burn, it should work (but even that is not fool proof). Often, the burner program will verify the burn, as good, as long as it can read it once in a hundred tries. You can hear it doing that, if you listen. Of course, it may not always work when you try to read it again. With audio/video media, it doesn't really matter if there is a glitch. With pure data, it does matter. Slowing down the speed, might prevent dust balls from floating around, but it won't work around a bad drive, or media.
FWIW, I used one DVD-RW for 18 beta versions of ArcaOS, and never had any hint that there was a media problem. One "interesting" thing, that I did see, was that i could see lots of small marks, on the surface, when using a florescent lamp, but I couldn't see any sign of them using a LED lamp.