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One of the myths of today's perpendicular thin film media is that there is no noticeable gain in medium signal-to-noise ratio (SNR) as grain size reduces below 8 nm. A recent experimental study shows that intergranular exchange coupling exhibits an exponential dependence of the oxide grain boundary thickness for the thickness below 1 nanometer. In this paper, we present a systematic micromagnetic modeling analysis regarding the effect of spatially random intergranular exchange coupling due to the variation in grain boundary thickness. As oxide boundary becomes sufficiently thin in small grain size media, a distribution in the grain boundary thickness is found to cost significant SNR loss according to simulation results.