Only so-so, not great.
coach
First, about the plaster component. FAIK, and I heard this from someone who worked a long time inside the factory, there is an additive in the gypsum mix in Greenboard, and that is the only difference in the plaster part of the product. It helps keep it rigid longer when damp, wet or waterlogged, before it ultimately falls aprat liek regular plasterboard sheetrock. They all lose rigidity and start disintegrating (falling apart) underneath the paper coating at some level of moisture and aging. The missing factor in your case here is how much the average permanent humidity is or will be in your shower. Not the peak 15 minute spike during and after a shower, but the real amount of long-term moisture in your environment. Not to worry. I don't predict any problem.
Second, about the paper facing. The product still has a problem, and the manufacturer knows all about it, and they have already started making another ("better") water-resistant board, that claims to be able to resist mold colony growth better. Moisture enables mold to start eating (ie. rotting) the paper, inside walls and ceilings, on the back side where you'll never see it.
The fact that Greenboard is more rigid when damp is not a plus when its bigger problem isn't solved, i.e. letting mold grow. All it is, is a big sponge that remains rigid longer than the low-grade product.
Whether it is screwed at 12" or 14" o.c. is not a big issue in the first few years. When it will sag depends more on the average permanent humidity in your environment (home), and 14" apart support is just 15% less mechanical support than the recommended amount.
David