same problem here: in xpacker my trees have a collision column 20cm wide and 2m high and I check the box. In btb I check the "collide" for some tree objects, but the car then collides with the whole tree mesh!
Also: in previous xpacker it was possible to set the "collide" property as default for objects, now it is not available anymore?!? (but you can do that on the xml file if needed)