I don't think buffer underrun errors are much of an issue anymore, or are they? At least not to the degree when CD burners first showed up. 2MB buffer is pretty standard. I don't know how much of a real world difference having a larger one makes.
If I were you, the main thing I'd wanna focus on is the burn quality. Color of the faceplate, LED colors, buffer ... they seem quite trivial if a burner can't perform to the standard you're looking for.
If you're satisified with the general sense of the burns for particular media you're targeting, then the overburning, overspeeding, etc ... seems to be gravy.
I guess it's a matter of perspective. But, I came into the DVD burning realm with the lure of burning a full DVD in under 6 minutes @ 16x. Then, thanks to this site, I learned quality of the burn is of the essence ... ie. having a 6 minute dvd burn does me no good if it turns out to be a coaster, etc. So, now, I don't mind waiting an additional minute or two to burn at 8-12x and give my chance the best chance for a high quality burn.
Just my $.02.