Hi Kiril,
Thanks for the reply! Thankfully, we're not otherwise using opacity in this case, so we can use it as a surrogate for visibility.
If I may say, I would recommend that you all add a more explicit note to your Diagram Virtualization documentation (http://docs.telerik.com/devtools/wpf/controls/raddiagram/features/raddiagrams-virtualization) saying that you use the Visibility property on items to achieve UI Virtualization. I understand now that's what you all are describing in the documentation, but it didn't register at the time.
Thanks again!
-David