In theory there could still be room for improvement. Right now ZBar estimates finder and alignment pattern locations to quarter-pel precision, but rounds each module location to the nearest pixel so it can sample a binarized version of the image to decide the value of that module. At the extreme limits of small module pitch this effectively turns the resampling filter in whatever you are using to resize your QR code image into a nearest neighbor filter. You can see why that would start to cause issues. Imagine a version 7 code (45x45 modules) sized to be 80x80 pixels. Most of your columns will be 2 pixels wide, but with binarization, somewhere in there you have to have 10 columns that are only 1 pixel wide. Good luck lining up your grid to hit all of them perfectly (without looking at the timing pattern, which would likely only help in the perfectly axis-aligned case). Some kind of sub-pixel integration of the original image before thresholding to decide each module value could probably do better. That would make decoding a lot more computationally expensive, though.
