The best oil is Silkolene Pro-S 10/50 and use a genuine Mitsubishi filter, chaep oil and filters do not work.

Before considering more power, make sure the engine compression pressures are good, should be 150 psi, minimum of 134 psi.
Ensure the full cambelt service is within date.
After that get it data logged to see what's happening with the fueling, timing etc.
Then if all is well consider the route to more power, of which there are several options but do not be tempted by the cheap bleed valve system, it will destroy your engine.
Welcome aboard.