I've never done late sugar additions and have never had a beer fail to attenuate. I think maybe if you're abusing your yeast, underpitching and underaerating, then yea, maybe it could make difference. But I treat my yeast nicely like you do and have done a tripel with 30% sugar and it attenuated fully.
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Monticello Barleywine, Red Nosed Tripel
Cali Common, Imperial Common, Sunshine Belgian Rye, Sticke Note Alt
Cherry Blackberry melomel
Belma Pale Ale, Cluster Fug IIPA, American IIPA v1.0, rauchbier, roggenbier