Opera max certainly is a battery hog.. bought galaxy j5 a month ago samsung gave the UDS update with Opera max since then Phones system started consuming more than 1 gb ram resulting the device to lag and draining the battery rapidly.
I installed this app many moons ago in my old Atrix 2 and then I simply forgot about it, might have to give it another shot.
I don't know how much data you can save but this app is battery hog. It not worth it crap.
This app works superbly in my phone. Saves almost half the data and makes 2g usable for browsing. But don't work on encrypted connections.
I found that the heat was from some apps constantly fetch the data through VPN in the background. If you block those, this app works fine and saves your battery.