If everyone using the same .WLD file it will work for everyone. You can play with modified .WLD files online, but it will cause strange stuff.
For example: Driver A has the fix barrier .WLD file, while driver B doesn't. Now, when the A hit the barrier it on his screen looks ok and doesn't send him airborne, but on player B's screen the car will still look like it flies high for a second before the sync is established correctly.
It's the same as car modifications, you can drive them online but the others won't see them - but feel them if they hit you (hint, spacetrip).
I think if someone use dirt track modification, it will look horrible laggy on the screen of the others. It will probally work but again, there will be big sync issues.