Higher than Sanji seems unnecessary. Atleast Jimbei being higher leads to a gag, Sanji and Killer have no interactions and the gap between Zoro and Sanji in bounty is quite small so no need to squeeze both Jimbei and Killer in there, Killer works fine being below both of Luffy's Wings. 1 billion sounds alright though I agree he didn't do much to garner it compared to Zoro who didn't exactly skyrocket past 1 billion himself. But he's the best Kid has so 1 billion to Kid's 3 billion should be fine seeing as they're partners.