I'm all in favor of anything which will reduce the time a fire can burn. And personally, I'd rather lose engine power than try to stop a ball of fire and smoke coming from the engine compartment. But the manufacturer has some limits that apply to this.
Cost: $120 each
Lifespan: 5 years
Environmental limitations: -40F to +185F
Activation: >200F or open flame
Given the above, I'm not sure it would be appropriate to store it in your engine compartment unless you have an electric vehicle..
But definitely post if you do use one, I'd love to hear how it holds up. The 'activation' at >200F was as little ambiguous on their website, I interpreted it as best I could.