Well, non-standard packages can cause some problems, we see a lot of problems relating to that with PHP. Also, we don’t do any testing with software versions that aren’t provided by the distro.
If you’re interested in using a different PHP version, my suggestion would be to setup a test server, and to try the upgrade there first.
Then, if that works, you could try it on your live server.
Well, we always recommend using the package versions provided with the distro you’re using.
However, the bad news is that those package versions don’t change. Wheezy will always have the PHP version you’re seeing there.
So if you really need PHP 5.5, you’d have to perform an upgrade; but we don’t recommend doing that But if you need PHP 5.5, there isn’t another way… so we’d just recommend lots and lots of testing before changing your live server.