The JL has certainly been tested by FCA with all factory bumpers to ensure correct airbag function and energy absorbtion to protect occupants. It hasn't been tested by IIHS or NHTSA yet, but will be soon.
However, aftermarket bumpers usually are not tested, and there is no data available on how they work in harmony with the airbag system and engineered crush zones on the Jeep to protect occupants in a crash. So, if you go aftermarket, you're assuming a certain level of risk. Of course, that can be said about tires, lifts, etc. also.
"If you wanna' play, you gotta' pay."
I do like how the wranglers design, by the nature of the cabin being narrower than the wheelbase, naturally deflects the vehicle away in that type of small overlap collision."Anybody know what the accident protective difference between a stock plastic bumper vs the steel bumper package would be? Anybody have front collision stats? "
As as to protective difference it might be best to break that (no pun intended) into two categories: damage to the rig and damage to its occupants.
There are a trillion variables to a collision, one of which is the tradeoff between the strength of steel, and in its inherent likelihood to more deflect, rather than absorb the energy of a collision, not necessarily a good thing: as that's energy's coming for you, and its ability to protect from further rig damage.
Contrast that with a more giving bumper which gets destroyed, along with much behind it, that takes the blow and absorbs energy your body, and that your passengers don't have to deal with.
Take this Volvo which features more and more rigid design as the passenger cabin is approached in the otherwise partial front impact test, where no engine blocks the passengers, and auto makers hate. This is a beautiful thing. Not because I enjoy seeing vehicles destroyed but rather passengers living to tell about it. Since this slow mo has no sound, I've chosen the second link for you to simulcast, both queued appropriately. Get the second video going first.
And of course, there's probably been plenty Volvo vs Wrangler accidents where the former and its occupants incurred more damage, if for no other reason than in sedan vs SUV, all other things equal, SUV tends to win.
And who wants to take a Volvo off road or customize it? It won't get into an accident there because it won't move there.
In fairness, the JK, without side safety air bags no less, didn't so bad on this one.
I cannot comment on the warranty issue because I don't know. I don't think it would affect warranty, since it doesn't really have the ability to cause other parts to fail. However, it may affect insurance rates. Grill guards are commonly referred to as "damage multipliers" because if you hit somebody, that's all the grill guard is good for...multiplying damage to your vehicle and to the one you hit. So that might be worth looking into.Could changing the bumper to an after-market void the warranty? I'm considering a grille guard, but it looks like I'd have to swap out the factory bumper, and Mopar doesn't have much to select from.