As Tobias wisely noted, whenever you add (big) power to a car you invariably lose some MPG. Granted there are better power/gallon ways to get it. Some of these can depend on how you need that power. If you only need the extra power very occasionally but "when you want it, you want it NOW" go for NOS. Its easy, cheap (compared to other alternatives), and provided you don't run more than the driveline can take, is fairly safe. If you want more power all the time, all rpm bands, etc....either a supercharger or displacement bump are the only good alternatives. For more top-end power and reasonable efficiency, I would consider a turbo kit. Unfortunately I do not know of any turbo kits for your application, but you may be able to adapt a kit for a 190 2.3 8v to work in your car. You'd surely have to get some custom bits done, but the 190 kit would be a good starting point. The only other thing I could think of would be looking into finding a Roots style supercharger and doing your own kit. Then again, if you would like more luxury, zero engineering/headaches, more power, and keep resale value intact just trade up to a faster factory model.
Hope this helps,