Probably,they've got a weather specialist among their crew,like Nami in the SHs, she's currently learning stuff about the weather, one can assume that with a certain level of mastery, one could manipulate the weather conditions to some extent,in order to maximize the speed one travels,I actually believe that Nami,after this current training arc, will come back with some techniques of wind manipulation, and will help them sail faster, a similar case might explain the speed of shanks crew,they are one of the 4 strongest crews..(if you still count the WB pirates)
That was only an assumption, a more diplomatic aproach may have been used to avoid a meating with Kaidou that could slow them down..
As for Shanks as a character and his influence to the story, only Oda'll know what to do with him,but now that Ace's died,luffy's had the taste of reality he hadn't had to this point,so killing Shanks just to teach luffy a lesson about life seems too dramatic and excessively depressive, if you consider Oda's work so far, still, eventually dying in a battle against Blackbeard,for instance, seems to cliché, the antagonist kills off the main characters idol to make him want to fight him without holding back..I don't know,Blackbeard's been responsible for Ace's death already,as well as WB,wich I think luffy came to like, so that stuff's been made already, although in a different way than usual,wich I liked,now luffy hates BB,but it wasen't that same old battle where he kills off the main character's friend and so he swears revenge..
There's just so many ways Oda can use Shanks that we can't predict accuratly what he'll do in the future,but I have to say I'd love to see a match beetwen Shanks and Mihawk before he dies or retires..