Nintendo is probably the worst when it comes to thwarting hacker attempts. They also are pretty soon going to drop the wii u and make way for the next gen. 5.5 is as exploitable as all other firmware. 6.0, 7.1.5, doesn't matter Nintendo made two mistakes with the console for which software will not fix. Once these guys find them (If they haven't already) it won't matter what gets patched the holes will always exist. I am by no means telling you to upgrade but in reality you are already past the public exploit so there really isn't any harm. Because there is a few private exploits that in fact do exist you really shouldn't worry. If I were you I would go ahead and get to 5.5 because it will not be too long before a release that works on that firmware is revealed to public. Just my two cent on that last part.
If you cannot wait til something is released then follow the post above mine.
Emceekarser I love you. You even made it clickable.