but my area is 10x10km or so, so the memory hit would be to great...
Which is why you would usually opt for both. You apply displacement map and it is being recalculated each frame based on pixel size. So the displacement would only be "active" in the first 100 meters or so, then it would be replaced by a bump/normal map. This way you get the best of both worlds and is standard workflow afaik.
Corona tutorial on it here:
Also, I'm not familiar with PheonixFD, but Ocean surfaces require an animated water texture to look convincing. Real flow provides such as an export. From demos I had the impression Pheonix did simulation only.