You wouldn't get much benefit in FSX, the reduction is size only matters with FS9, that can't load textures in background as efficently as FSX.
You will obtain the same effect, by putting down the global texture slider one notch down, which will force 1024x1024 textures to be used at 512x512
I was wondering that myself for jfk, I had forgotten that you can just put the slider down on global texture slider. unfortunly my video card is old now and its taken a beating and it is severely cutting my performance with only 256 megs of video. but will lower the texture slider thanks for the info Umberto.