Kitty hair will crack out, the flexable filler that Keith reccomended from Urethane Supply is the way to go. (And they have other products that will make the repair easy like "uniribbon" repair plastic you melt into place with a hot iron.)
Personally, I like the 88 grille/bumper, but if you must have a 89+ then just paint the grille black if you hate it so much. (It is there to somewhat protect your FMIC/oil cooler/Condenser/Radiator...) Imagine hitting a small rodent like a rabbit or house cat. With no grille, your going to immediately impact your coolers. With the grille, you stand a chance of dispersing the impact out into the grille, and even if it breaks, it might save you replacement cost of a cooler or two.
The photoshopped black grille looks nice. (I'm not saying the removed grille that have the bumper repaired where it was cut out is not good looking, I just think they are not as protective.)
Air flow difference would be minimal at best, and it's silly to even try and compare them! LOL
I would think things like the undertray and other air routing skirts and baffels make a big difference v/s cut out or uncut out grille's.