Just give it the "locust" ability (Rawcode: Aloc) if you want it to always
be able to walk through trees/destrucables.
If you want it to be able to/not able to under different conditions, use these two commands:
While it has no collision, the unit can walk through things, but will itself be treated as an obstacle.
Raydude, your solution (while effective) is horribly inefficient.