I think the best mods are suspension mods... the average customer bitches to no end if a car rides too firm, so carmakers will detune a suspension to the point that you can really get some amazing performance with little investment in a stiffer ride. A car that handles really well will definitely be more fun to drive... way better than if you sneak in 5 extra hp with an intake. I guess, a short throw shifter will even help you here (in the fun-2-drive factor) - although I remember those being somewhat expensive.
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