sounds good. let me know how it turns out (I’m about to start authoring on DVSP soon)
Your bit rate maximum should not exceed 7,500. Recordable media doesn’t have the high-reflective and error correction of replicated (metal) discs. Some of your end users could experience stuttering on playback if you go higher than 7,500 vbr.
If your average was set for 3,000 that explains why it looked bad.
The GOP changes will definitely keep the quality higher as the image becomes more complex. (everything moving) other than maintaining quality, the picture itself will not really improve and the filesize will be a bit larger.
DVDSP itself should not have any affect on the picture quality. It just brings it all together with metadata for compiling.