15x's turnover is a little too much flow for a sump to handle. At this rate, the water would move through the sump so fast that the skimmer wouldn't have much time to strip the water of DOC's, and there is a good chance of microbubbles. Plus, 15x's turnover on a 120 gallon tank would wreck havoc on a refugium if you planned on having one.
Look to moving about 5-7x's your display volume through the sump and use supplementary means of flow (power heads, closed loop, surge devices, etc.) to acheive a total turnover of 15x's (which is a good goal for a reef).