The wheel size alone doesn't effect your RPM or speedo. The overall diameter of the tire and wheel is what you need to look at. The overall diameter of the wheels/tires you have now is almost the same as stock, so your speedo/RPM is not affected by your current setup. If you switched back to stock 14 inch steelies, or the 14 inch HX wheels, the outer diameter would still be the same as your current setup. Even though the rims are a smaller diameter, the tires are taller, resulting in the same overall diameter. This calculator should help illustrate the concept:
Tire size calculator
I vote to trade for HX wheels with tires. The HX wheels are lighter than your current wheels, and lighter than the factory steel wheels. Plus, they look a lot better.