I'll freely admit to being one of the players who tends to know monster stats off the top of his head, though not to the point where I can recall every saving throw or BAB progression. I've never really had a problem with knowing too much about the monsters I was fighting, but then, I'm very careful to keep OOC and IC separate. In fact, in one game a good friend of mine DM'ed once, each of the characters had VERY little prior exposure to monsters. My character started out being familiar with a couple of monsters, because he was from the frontier region of the land, but even then we're basically talking about goblin and orcs only.
Even so, my DM was and is a big fan of not keeping monsters as written. He was very fond of giving basic Humanoids levels in PC classes, and better equipment. Our party once walked into an ambush set up by a bunch of Hobgoblin mercenaries, and let me tell you that was quite a battle.
We discovered a secret door, activated it, and came face to face with a Hobgoblin wielding a reach weapon. On top of that, there were two archers in the back corner who had been expecting us. The lead character took a hit from the lead Hobgoblin and two arrows in the surprise round, and because of the narrowness of the hallway it took a round or two to take it out so that we could rush the archers.
So yeah, I can see why it would be dangerous for players to know too much, but that doesn't mean the DM has to let them obtain the knowledge necessary for it to become detrimental. If the DM wants to give orcs SR 30 and perfect flight, then that's his prerogative. Unless they suddenly manifest these abilities mid-fight in a desperate effort to make the battle last longer, or any and all lore gathered on orcs previously never mentioned this ability, I don't have a problem with it.
As far as your characters know, that's how they've always been.