I think your calculation is highly sensitive to your assumption that 40 db is sufficient. If you run the calculator at 40db you get 50W as sufficient.

However at just 43db you need 99W and if you tried to avoid clipping and used 50db, 496W. Once you hear it, clipping distortion is highly audible because its so annoying.

I actually like this AllegroSound calculator better because it factors in 5db room gain by adding it into the speaker sensitivity and uses peak SPL at the highest end of the range in the calculation.

http://allegro-sound.com/Power.html

John