That's true that a "letterbox(16,16,16,16)" won't have any bad impact and will increase the compression.
I forgot to use it in my test of DVD-RB .
But anything went correctly even without that : Q found by new RB-OPT was 17. Anythign is PERFECT

Thank you Sansgrip to have done the mod in RB-OPT !

Not unless the source is very long or very difficult to compress.