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.
I can't be arsed to keep up this list of what's in the fermenters, but hey, check out the cool brewery I own!
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