Yes, it implemented. We find an FSharpSymbol under the caret before opening the Rename dialog, not it's usages https://github.com/fsprojects/VisualFSharpPowerTools/blob/master/src/FSharpVSPowerTools.Logic/RenameCommandFilter.fs#L92

I'm not sure why renaming without saving is faster (if it really is). However, having Highlight references worked on dirty files, it may sence.