The rated DVD speed is usually on the container of disks, sometimes on the disk itself in very faint lettering on many brands (the complete no-names may not be marked assume 2x or 4x max for those)
when burn to folder, you get the full disk structure, the menus etc are inside the various vobs. you point imgburn to the folder (video) for some unkowable reason the audio folder just seems to be like the appendix, no one knows what it does.
the burn speed is in that window (looks like 6.0 in your posting) you can change it when that window is open. I do not beleive you can permanently change it in preferences, you have to do it each time.
As I mentioned, i have had some problems with smartfit, seems to work with most material, but every once in a while it doen't like some footage or special effect. You can try "producing" it first then just putting that on the time line and making your menues etc.
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