The nice thing about spiral eel grass as compared to other species is that it doesn't get very tall. Only about 3 feet height and that is huge for it while more common species can top out at over 6 feet. It is the reason it is so recommended. It is also not overly invasive.
Whether it will out compete primrose, I doubt it. The grass carp will eat it and help control it, perhaps too much. It will mostly stay in the 2 foot tall range in my experience with it so far. Doesn't get overly thick allow you to fish through it and predatory fish to hunt in it. Check with Bill Cody and see what he has to say.
