Oh, looks like the GC allocation there is from string allocation when naming the shared materials. That's an easy enough fix...
At the bottom of SharedMaterialData.AsMaterial, try removing all the code related to the variable "materialName". This existed just for debug purposes and I had forgotten to implement it properly, so removing it should remove the problem. I'll try to replace this with a StringBuilder as it should be for the next update, or only have it be enabled in Debug Mode, where the material names are visible, anyway...
Additionally, the next update has some code that calls "OnRectTransformDimensionsChanged" less and adds a manual toggle to completely disable this callback, so that should increase performance too.
EDIT: emailed you a build with these changes!