Having lots of grass in a scene is always a tricky job because it may slow things down a lot. I have developed a method that is positioning grass quads around the camera automaticly, where their rotation etc. is reproducable (rnd uses world location as seed), together with EntityAutofade. This way you can have very dense grass on a very big terrain.
I would also reccomend to develop some kind of Grass LOD system.
I would also reccomend to develop some kind of Grass LOD system.