And I don't think they've had an adverse effect on fuel economy either - even when we have had warmer days.
This is to be expected to some extent, because the magic ingredient for keeping rolling resistance low is the same ingredient used to keep the tread soft at freezing temperatures: add silica to the compound.
For this reason, I'd expect LRR summer tyres to outperform performance summer tyres in certain winter conditions...
Performance/sport tyres use more rubber and little or no silica in their compound, which is great for grip when temperatures are high but also means the tyres go very hard at temperatures below a few degrees, causing traction problems.
LRR/Eco summer tyres won't go hard as quickly as sport tyres as temps fall - because of their higher silica content - but they don't have the special siping and "self cleaning" qualities of winter tyres, so can't compete on snow or ice.
I guess it's the siping and aggressive tread that makes winters slightly less efficient than summer eco tyres, but it's a close call as you say.
I've noticed that my winter tyres' efficiency is far more sensitive to wet weather (than LRR summers) though - if the road is wet it costs several mpg for the journey.