In various Forums there is currently a tip to install a carrier update on the iPhone 5, which was provided by T-Mobile USA. Due to included optimizations, the battery life should be improved. Furthermore, two icons for telecom services in the USA will be installed, which can be deleted. The update can also be installed on German iPhones, but has the problem that the APN is not the right one.
If you want to try it out, you can download the corresponding Mobileconfig file at tmobile.com/isetup. With the iPhone configuration program the APN can be adapted (in the German Telekom network "internet.telekom" is the correct setting, which also applies to Congstar).
A customized version with a working APN and without the links to T-Mobile services I have finished configuring - if you want, you can download the download here. I do not take over any guarantee for possible damages or necessary new installations - the installation of the profile is at your own risk.
Whether there are actually improvements regarding battery life or other changes I will only be able to say in the coming days - if someone has experience, comments are of course welcome here.