I have UT and GEX installed. It has had an effect on FPS but simply by dragging them down into the thirties. No big deal.
I installed on top the Victoria + scenery and I got hammered. FPS down into the 15s.
Sure I'm running a quadcore at Very Dense/Very Dense, but a hit of 20fps might suggest something about the scenery design.
It's important for designers for FSX to recognise that big hits by scenery or aircraft really hurts in this game. In my case I can turn the settings down, but it's fascinating and worrying to see default aircraft and default scenery and autogen give a return in the 100+ FPS and then see it knocked back with a hammer as the add-ons pile on.
But UT and GEX is reasonable, Victoria+ is too big a blow.