Although the RS engine is based on the 1.8/2.0 zetec-e, the RS engine is mechanically quite different because it has a turbo. The requirements for a normally aspirated engine and a turbo one are very different so there's nothing you can take from the RS that will work on the 1.8.

Adding a turbo to an engine transforms it's ability to be tuned so yes, the RS engine will give you massively more power per pound spent. If you're going to upgrade to an RS in the next couple of years don't bother tuning the 1.8, it won't be very sensible financially.