Yes, the grill will be 3/8" standing off from the woofer's furthest exursion point. so the drive will not hit grill cloth, when in motion.
Diffraction is minimal since were only talking 3/8" difference. After many hours of listening, the difference with grills on and off is hardly noticable. In regards to my efforts building these grills for added protection, and enhancing the appearance visually, I could'nt be more satisfied. Plus it's an investment option, that will enhance resale value with your NX-STUDIO's.