Well a couple of things. I have heard of frame resonance in some speakers so if that is the case then wrapping just the frame spokes in deadener may help. I doubt this would make a real world difference in a car where the background noise is so high though.

Now if you are talking enclosing the back of the speaker in deadener you are basically creating a very small enclosure. Because the airspace is so small it works like a much stiffer spring. This would probably let the speaker handle a bit more before it bottoms out. It would not change the amount of power it can handle until the wire inside the speaker burns out. Anyways one of the other effects of a small sealed enclosure is that it creates a response that is much more peaky. It wont go as low but it may sound like its "hitting harder" as some would say.

I could see that maybe if there are many air leaks to where the sound can get from behind the speaker to cancel out with the sound coming from the front it may help even more. If you would like the sound like that i dunno. You could try it then if you dont like it then just cut out the deadener where it covers up the holes and no harm done. Just dont cover any ventilation holes that help to cool the speaker. Sorry for the rambling.