***I was hopelessly, utterly, and pathetically wrong, disregard***

His hardware solves 3.45 shares per minute, taking on average 17.3 seconds to find each share.
Statistically, that's just over 34 solved shares per BTC block.
Let's for the sake of clarity of argument (ease of calculations) assume 2 NMC blocks per 1 BTC block.
It translates to over 17 seconds of his work being lost, on average, during each BTC block.
That's nearly 3% performance hit due to merged mining right there.