Vastly different extrusion width between versions

I happened to slice a file tonight with 0.7.2b and 0.8.4. In the comment block at the top of the gcode file it lists the extrusion width. For 0.7.2b the width is 0.7mm. For 0.8.4 the width is 0.53mm. As far as I can tell the settings are exactly the same for both versions.

Has anyone else noticed this?

I've also noticed that 0.8.4 produces "wobble" in the print that looks a lot like Z wobble. It doesn't show up on every print, and prints sliced with 0.7.2b don't exhibit this, so I do not think it's printer related.

The calculation for optimum extrusion width changed with 0.8.

Ah, I see. So which one is actually "optimum" then?

I'd say the new one is closer, you really want your w/t ratio to be around 1.5, but less than the free air extrusion width and not much smaller than the nozzle width.
I usually explicitly set width to .5 for .3 layer height and go down to .4 for layer heights below that. But you'll have to experiment.
If the w/t is too low, the filament will break as it's extruded.
FWIW I had issues with all of the 0.8x drops, 0.91 appears to be OK if you never remove anything from the platter.