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.