Does filling your logbook give you anything special like a new gallery or something?

filling your log book gives yuo 100% completion and with that the 100% ending cinematic. that's what filling your log book does

I have 100% completion already but my logbook isn't completely filled. I have all of the power ups so that is why I have !00% completion and have gotten the full ending.

No, the logbook doesn't give you anything extra but having all the scans does help to expand the storyline. For example, the lore found on each of the worlds gives you an understanding about how that world developed and what happened after the seeds hit. It's really rather pointless unless you want to spend hours reading about the back story.