Is this your first trip?
For a party of 5, personally I couldn't do one room. I can't even do one room for the 4 of us any longer (but I know everyone is different)!
I absolutely love going last week of August. Most kids in the US are already back at school, so even though park hours may be shorter, the crowds are nothing like spring break or the winter holidays! Last August, Soarin' was a walk on all day during the week before Labor Day. Now, yes that weekend things picked up, but we left by then (my daughter's school starts after Labor Day).
That being said, expect storms. Every. Afternoon. So if you want a pool break in the afternoon, it may or may not happen. If it storms (and it will), and you planned on going to the pool, you'd be stuck in the room. I don't mind waiting it out in the park, or in a condo with space, or a room if it's just DD and me (which is is often). I did 15 nights once with just me and the kids, but we had an offsite 2BR condo 5 minutes away from Disney, so that was fine.
Have you considered renting a 2BR DVC villa?