He also asked about the bulkhead, which just doing what you suggest won't have much effect.
if you dilute the degreaser properly, you will not damage the plastics, after washing them treat them with some dressing product as suggested and all will be fine.
You don't want to blast the engine bay with a jetwash too much as there is actually engine in there just be sensible with it.
If you need more information then head over to detailing world, plenty of guides on cleaning engine bays.