I have Real Environment Xtreme (REX) installed for a long time but I didn’t aware that its freeware could bring additional enhancements to FSX until recently. I thought they were included in the package.
The Enhanced Grass Texture freeware instantly upgrades the grass resolution to a new height. (Edit: The company has changed its rule. To download, you must register and login to their public forum.)
Installation is as easy as snapping fingers. But DO REMEMBER to make a backup of the original grass texture file. Moreover, it may have barely perceptible impact to the overall performance.