I 100% agree with the cloak idea. I think it would be really useful to not give up your location so you could sneakily take the lead without anyone noticing or trying to block you
I also like the idea of missiles not being able to lock on as it would not only mask your visible signature, but also your heat signature as well (I am assuming that this is how the missiles track the cars in game. However, that being said, I think that if the car comes back out of this cloak before a missile has exploded and is still in range, it should be able to pick up the car then and find its target. This way, if you saw someone go cloaked, and you know how long it lasts, you can launch a missile tactically and potentially still hit your target when their cloak runs out.
This and I also think that the "cloak" should not be visually perfect, but create some distortion in the light traveling around it. This way, there is a chance that other players could catch a glimpse of the cloaked cars location (even if they can't do anything about it) so they can start planning what they will do about it. You would need a really good eye for detail to see it at these speeds but if you were really attentive, you just might catch it. Also, if there is a slight visibility to it, it would help on the player's screen with the cloaked car so they can still see the basic shape and not accidentally clip it on something that they didn't realize they would be hitting, thus ruining the race for them.
Just a couple of thoughts.