Grenades blow through almost every building material you can think of with ease. Grenades thrown in-doors are pretty loud even if you have some sort of hs/ld ear protection it's going to rock you pretty good.
I may have not been clear, yes you want it in the room. Generally you want in the fatal front area, yes it's called a fatal funnel for that entire situation/entry point (and all sorts of other names).
In this instance its the fatal front(in relation to that fatal funnel). Generally you want to be able to clear your nearside corners and the immediate area in front of the door. The point is not to chuck the Flashbang in too far or you are not going to get the full stun effect in areas with the highest threat.
If you are 6ft like me you generally pretend you are tossing it just to your head if you were laying down. It's going to roll/bounce in most cases and go farther. In any case you are aiming for a point that is most likely going to be open. Flashbangs don't work so well if they roll under a couch or a TV stand.
I guess our grenade experiences differ, If I requested a crate of grenades. I would get a crate of grenades.
If you wanted to reduce things down you want to get the maximum stun in the areas where you do your fan-out to slow down anyone who might get grabby or dont need to get a good sight picture to hit you. That's all going to happen in proximity to door. in an ideal world your going to want to hit closer to the center but in less you stick your head in first or have a god-like sense of the local building construction and layout signatures you might as well throw into an area that everyone can see the flash and for the folks close in the bang.