Dude where do u want to race your car? I have a city 2002 Exis and i think its already a pretty fast car.
My suggestion, when ever you feel like going a bit faster just convert to petrol. It has a 95bhp engine which is faster than xli, gli, baleno and 2003-2008 city. With the superb handling it provides you can cut steep corners at good speeds. Installing plugs is ok but don't mess up your engine by putting in extra stuff since your car as u mentioned is already choking. I don't experience any such choking, may be just get ur car tuned and get the engine condition checked . Getting all those extra mods will put additional strain on your 11 years old motor and you will end up rebuilding.