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.