@LaFF:
A bounty isn't going to make Luffy stronger. Think about it, if Luffy beats Doflamingo with his current moves we know of (Gear 2nd/3rd, gattling, elephant etc) that wouldn't make a difference, Jinbe would still be on top because Luffys actual power hasn't changed from when Jinbe and Luffy last met and only the reputation has changed which is beating Doflamingo (personally i can see Luffy beating Jinbe now, just other people think otherwise).
If Luffy pulls out some new powerful moves then yeah, to people he will be stronger than Jinbe.
When he beats Doffy it will be enough, it will show his increased threat to the WG and that he has grown beyond what he was when jimbei saw him last, that will be enough evidence to show that at the very least Jimbei is no longer stronger and that is what is required before he can join. Power levels are meaningless in this manga but Doffy's scalp will show that Luffy is strong enough to have someone like Jimbei in his crew, if you ask me I suspect that both Zolo and Luffy are stronger than Jimbei now on land (remember he fought Ace to a standstill on land pre timeskip), but they will never be as strong as he is in water which is why he is so valuable as a new nakama.
–- Update From New Post Merge ---
@Shadowgreed:
Who do you think was stronger at the war Luffy or Smoker?
At the war I would say Luffy would be stronger than Smoker as he was beginning to use his Haki even though he didn't really understand what he was doing, as we saw in many fights pre timeskip you don't have to understand haki to whack someone with it. Smoker's defense is pretty weak even after the 2 year timeskip (Smoker hasn't had much luck lately with his matchups tbh) so it would have been the same or worse back then, I know his fruit can disable his opponents very effectively but I very much doubt it would have worked on Luffy during the war.
–- Update From New Post Merge ---
@DarkFalcon:
the condition about Ace/ Smoker fight was what if Ace didn't have his fruit power. You know it's one of those "what if situations"
Well Ace still had haki and Smoker's defense is not that great so I would say Ace most likely, to be honest it has never been clearly shown just how strong Ace's haki was or how well he could use it, if we take his draw with Jimbei as a guideline I would say Ace would still win against Smoker as his haki would have had to be at a fairly decent level to go the distance with Jimbei.