Funny you ask that...I was going to ask a similar question. I started on Sunday the 24th with BRS dry rock, but live sand. My ammonia had a similar low spike (at least lower than I thought) even though I have a cocktail shrimp in there, and my nitrites are off the chart.
I'm guessing the live sand for me, and the MB7 for you, is what's getting the cycle started quickly. I think I'm going to wait out the full 4 weeks even if things zero out, just to make sure the rock is good.
BTW, I'm going to be in Harrisonburg sometime in June/July. Small world
Edit to add: I am still reading non-zero ammonia in the tank, even with the nitrites. I haven't added any bacteria besides what was already in the sand. (AragAlive)