Thanks Sanigo and Juju for speaking common sense to Jadosra. Anyway Jadosra my pt was that there's nothing that says Shanks got stronger in the ten years that have past. Now the reasons other than that fact that lead me to believe he hasn't got stronger has to do with a few things.
First I think that the NW has a tentative balance thats maintained by the Yonkou's power over their territories that divide the NW up, so I would imagine the only thing that keeps each Yonkou from taking territory from one another is that they all are likely close in power so any attempt on the turf of any of the others is deterred by the high casualties the attacking Yonkou may receive to gain anymore territory. This is backed by the fact that as soon as it was clear that WB would possibly be vulnerable while trying to save Ace that's when Kaidou decided to make his move. So unless every Yonkou is gaining strength equally over the same time frame I doubt they could keep any balance and if that were the case then they'd constantly be fighting each other in turf wars, which would make them all vulnerable to the marines as an outside 3rd party.
However I doubt they could all be gaining strength because I'm fairly certain that WB has been losing power as he has been aging, he may have been a lot stronger in his Roger fighting days and his rep and crew are what keep the other Yonkou in check.
Lastly, I don't think Shanks has gained any strength not only because of his arm but also that the way that someone usually would get stronger is to fight someone who can give them a challenge, having lost Mihawk as someone he can duel who else would there be? Lastly, had Shanks gained a lot of strength as those that think he became a Yonkou in the last decade believe, then I imagine he of all people would know that he has gotten stronger, and if he had then I doubt that he'd make such a big deal over a scar that BB gave him before the supposed "power up" he even went so far as to point out that BB didn't sneak attack him, so that's why I think he's been roughly the same strength for 10+ years.