Hi, we made several tests, but could not reproduce that difference between Edge and Point mode.
These are our results with one of the tests scenes (you can find it attached):
Edge Mode, Corona 1.7.2, 15 passes: 00:52
Point Mode, Corona 1.7.2, 15 passes: 00:45
Edge Mode, Corona 1.6.3, 15 passes: 00:55
Point Mode, Corona 1.6.3, 15 passes: 00:48
Edge mode is a bit slower but not much more. That's normal because Corona must render the hidden grass blades as transparent items.
Perhaps render time may higher using other materials, i don't know.
As said before, there is not much we can do, because rendering process is out of our control.
If the Corona team replies something new in their forum, please tell us.