All the indicators are there that tonight (with Kelly clarkson) will be packed to the gills, to a degree that happens very rarely. They are forming a special line to get near the stage for this, probably very early if you want to get anywhere close to the stage.
Valet parking will be nice going in, but the sheer volume heading out may take a while. Heck just getting out of the parking garage post concert with normal parking may take an hour or more. (Getting in if you arrive around 5 or later may take a while too, even with all the parking booths open) Add the fact that the Bay Hill Invitational Golf Tournament is going on (and they use Universal for parking for it) that will add to some of it, but not too much.
If you want to Exit during the concert, or after, I seriously recommend walking along the left side of Monsters (Where the mardi gras food and bars are) around the back toward Hollywood blvd to get out, don't even try going through the concert area. Or even by Shrek. They will likely open the big "Red Carpet Gates" to let people out (maybe even the seldom used entrance/exit to the left of Guest Services), but it will still be crazy.
Batroom wise, there is one by Twister toward the stage, one on the left. One by Mels, one by the Exit (near guest services) one by the exit near the turnstiles, and one outside the park that will probably be least crowded far to the left after you walk out the exit gates.