That is too much of a drop.  I lose very few frames.  I have a Dell XPS430 Core 2 Quad 2.5 with 6 GB RAM.

I use Alacrity to unburden my system before loading FSX, and control my Nvixia GeForce 9800 GT with nHancer.

I find that traffic and airport vehicles are the big frame chompers.

One thing I've found is that unlimited framerates give much better performance for me with this add-on.  Also, I always start with the default Cessna, turn everything off including the key and the switch at the gate.  I use the "High" scenery settings, with no traffic other than 60% X Traffic and usually Medium Airport Vehicles.

I can get 8 fps if I slap 'er down at O'Hare's active with traffic and airport vehicles.  If I'm careful, I can get well over twice that, and in the 30s after takeoff.

There is no doubt premium visuals are going to require a coolness tax in fps, but it is very small for me.  What kind of system are you running?

What else do you have running?  How are your FPS in other circumstances?