A complete answer of why the sceneries are not compatible with Instant Replay, is given here:http://www.fsdreamteam.com/forum/index.php?topic=1597.msg17467#msg17467
The texture size doesn't really have much to do with fps (unless your video card has less than 256MB) but, instead, with eventual stutters, and not in FSX anyway, that's why the resizer is made for FS9 only.
You can check this: by lowering the Global Maximum texture size slider 1 notch, it will force all textures to be not higher than 512x512 instead of the default 1024x1024 so, it's quite similar to use the resizer.
You can try it and check the effect on fps on your system. If the fps increases, then you should probably consider a better video card. If not, your problem is elsewhere, probably CPU-related and having set settings too high for your system.