I find it hard to believe that after the build up we've had for Merman Island, all the speculation, and all the anticipation for the Strawhats to finally get there that it will simply be BOOM headshot done with Jinbei. There are other ways of showing that Luffy's gotten strong than by downing a Shichibukai in one hit that wouldn't sacrifice a potentially awesome battle. That's not to say he will fight Jinbei, as I'd rather have Luffy not fight him at all rather than beat him so easily, but if they do fight it should be something that is epic and has a lot of emotional impact behind it. It could be a number of things but thats not important. The point is that OHKO against Jinbei would cheapen the whole arc that's been anticipated since Arlong's introduction.
Speaking of Arlong, that comment Yoska made was probably introduce soley to implicate what a threat Arlong was. At this point in the manga, what did we know about Shichibukai's strength? Not much, but we knew about Mihawk. We also knew Mihawk kicked royal amounts of ass. Thus, implicating that Arlong was on that level sets up a nice opponent for Luffy to fight. However, when the time comes to confront Jinbei, I'm sure Oda will find a way to renig on that comment and show Jinbei to be the powerhouse I'm hoping he is.
On a side note I'm kind of hoping he's a runt of a fishman. Not a midget persay, but just a head shorter than the average gyojin. I just think it would be a nice, funny moment for the unveiling of the legendary fishman only to reveal he's the runt of the litter.