It will help that much under FSX, expecially if you have at least a Dual core CPU. The main reason for having the resizer in FS9, is to overcome stuttering (not low fps!!), due to the fact that the FS9 graphic engine doesn't use the second core to load terrain textures in the background, like FSX does so, a big amount of data being loaded might effectively stall FS9, hence the stuttering.
FSX has a better texture loading mechanism that is using the free cores to handle the loading of textures, so stuttering shouldn't normally happen. The only one that might have a real benefit lowering the texture resolution in FSX, might be people with a Single core CPU and only 256 MB of Video RAM, but I wouldn't even suggest using FSX under this config to begin with...