While you're understanding of the equations is true, I think you need to look at things a little differently. Your first equation, yes as you increase current, voltage would go up, but how do you increase current, especially in this application? You are changing voltage, so you need to use a different variation of the formula, or plug in the voltage number you are using, and then solve for I or current. You will then see, as voltage goes up, your current goes down.
So as you increase BEC voltage, the current goes down. You can't change the current, you can only change the voltage, in this setup. Power consumption can be calculated, but for ease, just assume it stays the same.
Current increases in this application because as you increase voltage to your servos, they will operate with increased speed and torque - obvious by looking at the specs of any servo at two different voltages. If the servo specs stayed the same at higher voltage as they are at lower voltage THEN they would draw less current at higher V. The motors in your servos are NOT the same as a static resistive load and cannot be calculated as such. You cannot assume power stays the same as voltage increases because it doesn't. As you increase voltage to your servos, they increase their power output: they are faster, stronger, they can get hotter, and will absolutely demand more current.
But... it really doesnt matter as long as you have a stout BEC
Pick your servos and have fun!!! I run some of my HV servos at 6V. The specs are still great, and if you can feel difference between 6v and 8v you must be part machine. Congrats on the E5!!!
