Ya plugs isn't the problem.. 13 is in the norm.

Also, more power doesn't mean less gas milage always. Depends on many factors. My Mitsubishi Evo was getting 17mpg stock as an average.. now with 150 more hp over stock it gets 24 on average. Best is to have a tunable car and make the engine breath better. But ya the vx is fat and under powered so its going to drink. We get about 17 on hwy and 12 around town.