A way to maximize power is to reduce rotating mass and minimize moving parts.
Turn linear motion to rotating motion, replace push rods, rocker arms etc to a rotating motion to avoid the energy losses.
Look at modern engines with a camshaft directly driving a hydraulic cam follower on top of the camshaft, no rockers, pushrods etc. required....less linear moving parts.
If pistons traveling sideways has shown promise? Then why not take it farther?
That design was not made to reduce gravity effect.
The boxer engine was designed to reduce harmonic vibration in the engine by opposing the cylinders in direction.
The other reason was to make the engine lower, causing its point of gravity to sit lower, which helps the car's stability.
All in all the internal combustion engine has poor efficiency, i believe that even on modern engines 70-75% of the energy created is lost through exhaust and heat, only the remaining 25-30% is making the car move.
All the improvements the engineers had made over the years like forced induction, fuel injection, variable valve systems, etc. improved some but the basic concept cannot be altered.
Look at jet engines, a single rotary assembly that turns at high rpm, no other moving parts are there.
All auxiliaries like fuel and oil pumps are run separate to keep the beast in action, energy efficiency is a lot higher here.
