This reply is to an old post, but I've been doing some tests on strength relating to the top fill pattern, and on thin walls switching from the default rectilinear to a hilbert pattern on the top layer decreased the stress/strain measurement by 40%. In other words, it increases stiffness a noticeable amount.
I am use the Concentric infill pattern. For me it looks like the layer lines don't overlap each other as I think they should. I have tried Vase mode and that is exactly what happens. In Vase mode my parts just fail because one ring of filament is being printed to far apart from each other. I have tried upping the overlay setting to 20% but no change. In regular print mode I see the same issue with at least the bottom first couple of layers. Its just not printing layer line close enough together.
Concentric and regular printing modes should both still have their extrusion lines butted up against one another. If this isn't happening then some extrusion settings calibration may be needed.