Performance is not a factor here. 99% of the body kits (notice the spelling) are designed for looks, not drag reduction nor downforce.

If you're worried about chip and crack resistance, that's an issue of durability.
Which, if the car is a daily driver, obviously means urethane is the smart choice.

If you're adamant about molding in your kit, it sounds like you're building a carpet queen rather than a daily driver.