That's actually really good mileage.
2 things that are easy and free is to increase your tire pressure (I run 38 and noticed a big difference over the 28 that they were previously), and drive slower. Look at the instant econmy readout on your overhead console. There's a BIG difference in mileage between going 65 or 70, or even 65 or 60. I set my cruise on my ZJ to just under 60, I get the best mileage there. If it takes me a few more minutes to get there I'm going, so what.
You have to look very closely at any parts you buy to increase mileage, and ask yourself if it's worth it. If you spend a thousand in mods to get better economy, and get 1mpg better, how long will it take you to recoup that money in fuel savings.
And leave the cat on, removing it won't help mileage, and our air is poluted enough as it is.
And FYI, I only get about 14.5 with my I6 ZJ.