By the way I'm using m1, on iphone 6
m1 removed this option some time ago. this is carrier dependant, so some US carriers still allow this.
We still have 2/2.5 in singapore but i believe m1 is upgrading it all to 3G. 2.5G is seen as gprs on your phone. frankly, my battery drain is more significant when my phone is on GPRS and keeps scanning for 3G. 2.5G is slow in most situations, including reception for whatsapp and downloading info from facebook, outlook, gmail, and practically everything else (hence 2.5G).
4G is great and speedy, but it does drain a little more juice, and your phone may heat up a little. just keep your phone plugged in and switch to wifi when you're at home, and you won't have to worry about usage.
so back to your point: don't worry too much about toggling off 3G back to 2G like on your trusty iPhone 4/4s. your phone can manage the battery just fine.
p.s. i also replied on macrumors regarding your post