Im sure i have read tests and that tesco 99ron is the best you can get. thorny motorsport even sponsor the stuff. I never used to believe in the stuff but i really do think injector cleaners work ie wynns. Quality original parts like Iriduium plugs, fuel and air filters and correct oil etc will help and an overall fault diagnostics with measured air mass flow to make sure you mafs working would be good aswell. To increase an NA engine all id recommend a full exhaust from headers and a panel filter /induction kit. Maps are seen as a bit pointless on a NA car with lower BHP but they can crispen the throttle response and mid range which isnt shown in total bhp/tprque figures. Throttle bodies will simpyl cost loads and cams will make it thirsty and perform worse unless in the sweet spot but are very nice performance mods with huge gains.