I read through your code. Although your declaration is confusing (old,new,..? find better names) you should add more comments. And it is obvious that you missed to tell the reader HOW a grass model should look like to work with your script!

Your script tooks the model and "projects" it on the surface. Like when you have a soft rubber pad that is layed on a hand: it deforms as the shape of the hand. Like that your script does the job..

But the very reason why your solution is superior (on the one hand) is that you have got a lot of grass (sprites? polys? etc?) inside one model. So it renders fast cause it is only a small amount of entities instead of having lots of grass spikes

But it is also inferior, because even if you have a speed increase, it decreases on wide scaled levels and areas. This is because you have to have different shapes of your grass sprite to fit the flexibility of nature. Nevertheless you have a lot of entities and polys too. So you need nevertheless a more flexible system.

Search for "dynamic tree solution" here and try it with smaller mdls with grass bunches. This works fine!

ciao
christian