To be honest, there isn't much difference in the amount of metal present so I can't imagine there would be much difference. Probably more difference crashing with or without the spare wheel in place.
Most of the crash absorption is done by the bumper armature and the box sections which are bolted onto the subframe and these haven't changed.
Really, there would only be one way of knowing and I don't plan on testing it.
If you have any data or know anything that would effect it then please let me know, but to be honest, with most mods like this, you just have to go with rule of thumb.
I share your concern if I was removing a lot of metal, or adding a lot which could make the front end too weak or too stiff, but I'm not. Essentially, all I have done is removed the wheel well, put it back in 180 degrees out and added the single plate of metal to direct the air.