Personally, I don't see how the fuel rail covers should trap "that much heat", I guess if you can remove any "obstruction", and let cool air get into that part or place, it would help performance wize.
I dont think it would serve any significant purpose. When you are moving, fresh air is being circulated though there keeping temps down. Just like the whole situation with the CAI enclosure - the enclosure wont make a big difference when driving b/c of the fresh air constantly circulating. CMNT talked about this in one thread.
Dudes (and Dudettes), you should see the amount of heat that escapes through the louvers in my carbon fiber hood. Makes me damn glad to have it. To think all that heat is trapped under the stock hood... (shudder.) One downside, though... at high speed (120 MPH) the carbon fiber hood sounds like it's going to shake off the hinges. The heavier, more secure stock hood wouldn't make a sound.
Anyway, there's definitely a lot of heat under the hood.