Well, you might want to write "the only pilot in the whole FS community" instead of "one of the many" on this case, Pete :/
Unfortunately, on all FS forums with no exception, everybody has ever complained about the terrible performance of FSX (and P3D) over large airports or special sceneries.
Of course, if you let your simulator completely empty (no additional sceneries, no AI traffic, light weather), the impact on the FPS when coming around an airport remains confortable.
If you do that again with a serious aircraft, the story is going to be different.
If you do that again with a serious scenery, the story is going to be even worse
I too can get good frames, like 40 to 50, over the default Seattle airport (which is one of the worse places in the default world)... and as soon as I get away from it with the same settings, my FPS raise up to 100 FPS or more. There definitely is a dramatic impact, 50 to 60% les FPS in that case. Sure, it's stlil flyable, but nobody want to fly in such conditions nowadays.
Concerning Dovetail new sims (there will be two of them), the only one which won't allow addons will be Flight School if I understood correctly. But that one is not for the simmers. The target audience are the casual gamers or non-simmers which would like an easy experience to discover the world of simulation. It's nothing more than an entrance door.
The other sim, DTFS (DoveTail Flight Simulator), is supposed to be open to addons. I don't see where anybody mentioned there wouldn't be addons ?