Fuel system, fuel quality, temperature conditions and surging issues, plus quality plugs and a well maintained ignition system.

Have and will always recommend an AFR meter as a first mod on any performance vehicle.

Another Impt mod is a water temp meter. The engine/water temp needle from the middle of factory meter on Jap vehicles are scaled unevenly. The middle to the H top measures 15deg. Edit: slight temp variations are not shown on the factory meter.

The common ear is not attentive to pinging, so the above, plus a knock meter will be very useful for a tuned vehicle.