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If we go for regular 32 colors and keep the first 16 for background, we can do a lot of optimization to speed-up blitter operations
- turn on-off 5th plane above and belove players
- do fast blitting into 5th plane
- do fast clear on 5th plane
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Yes, there are many benefits when the background and Bobs have their own 16 color palettes.
In Blitz Basic it's possible make 16 color blits to a 32 color screen. Normally this would cause strange color effects, but for the Blit command there is a flag called "ExcessOnOff", which I think prevents the blit from modifying certain bitplanes. I have tested it, and it seems to work; with it I could make 8 color blits to a 16 color screen without messing the colors, so 16 color blits to a 32 color screen should work OK too.
The main benefit of this is that all character graphics in memory can be in 16 colors, instead of 32 colors, which saves a lot of Chip RAM. And also they take less space in the floppy disks.
And one other big benefit is that the 16 background colors can be freely changed with the copper, without worrying about messing the BOB colors. This is one "missed opportunity" in the Us Gold version; it would have looked much better if they used copper for the backgrounds. A gradient for the sky and the ground + lots of extra colors for everything else, it would easily get at least 32 colors more.
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And about sprites, I would use them mainly for the background animations. Four 16 color sprites can cover many BG anims, leaving only some for the blitter. And the energy bars and the timer I would put to a separate screen area (maybe 1 bitplane only, bars colored by copper and sprites for the timer).