As per an actual WWII pilot who flew a P-47 (Wonderful Winnie) the "100% hard" flight setting is absolutely ridiculously, unrealistically difficult in CFS2. He refuses to fly at this level.
I also flame those who fly at that setting because of this absurd level of overkill by M$. It's not, in my opinion, even worth the effort to fly in because it is so unrealistic. The closest thing to realism, as per the WWII pilot (RAF_Yank), is the "95% medium" setting.
Granted, flaps only add drag, the planes aren't accurate, as the hell cat has better range than the corsair at their cruise altitudes, among other issues, but the fact remains for realism, 95% medium's the way to go.
On another note, I actually took the time one day to fly all of the stock flyable planes as far as i could to determine their ranges at their respective cruise altitudes. If anyone would like a copy of the numbers, let me know.
Edit: Don't get me wrong, I enjoy CFS2, but it really feels like M$ could have done a LOT more with it. It's almost like CFS1.91a, not CFS2.0