Your questions are a little complex but I will attempt to break it down and also link you to some helpful documentation. UE4 supports iOS and Andriod devices so the games which are optimized for mobile should run with varying levels of speed and efficiency depending on your chipset. Here is a helpful list of supported Android devices. And, here is the list of supported iOS devices.
Both supported device lists reference our performance guidelines, which can be found here. This is a great place to begin dealing with mobile optimization, but as far as how to “optimize it more,” that is entirely a decision that must be made by you or your design team. As you say above if you are focusing on the xperia-p (which I am sorry I do not know what chipset that is), you may want to remove certain things from the game or combine certain textures into atlases to reduce draw calls in an effort to make the game function as you expect on your target device. It is a tightrope walk though as you may reduce or change certain items for one older device only to have it look not as optimal on a different newer device which might have more memory or a larger processor.