Esp. when carrying more people with AC and low RPM.
Side gap + regapping your spark plugs will help to improve performance by quite a noticeable amount. My original gap is 1.1mm and I increase it to 1.4mm. The difference is very noticeable. Tried with 4 people + AC and under hot day, the car performs well. I can't really feel the extra weight of the car at all. Even going up slope is not a problem (mine is manual).
Of course, you have to experiment whats the max gap for your car. If you experience misfiring under WOT at high RPM, this means the gap is too big already.
One cheap solution to get reduce misfiring problem is to install an ignition booster. Not those VS, its those which boost the voltage to ~16V. With slightly more primary voltage to your coils, you will achieve a higher secondary voltage to your plugs, hence resolving the misfiring issue (of course there is a limit as well).