Will burning a DVD at the slowest media speed result in a higher quality burn or does it produce the same result as burning as the highest supported media speed?
Depends on the drive & the media. Different write strategies are used at each speed. In general, slower is better, but you also need to take into account the effect of the different write strategies. There is no clear-cut answer; it’s a matter of what works best for your drive using your particular firmware version with your media.
Here you can see the difference with one media. Of course that’s only one test with both speed so there can be some variation in the results if I make more tests. (first test disc faulty?)