A follow up my last post on figuring out Google+ Photo storage: https://plus.google.com/105044448821136870719/posts/P6CL3cLqJnT
I used Takeout to download an archive of my Google+ Photos. For some reason the download always failed at the very end but this represents > 99% of my data. Using 7-zip, Duplicate Files Finder, windows advanced search syntax, and a bunch of sed and grep I found that:
The archive contains 240 GB of data, about 100 GB more than is reported in the storage report.
I have about 60 GB of short (<15 minute movies) which shouldn't be included.
I have 34 GB of duplicate images (26 GB duplicate videos already discounted as they are short).
I have about 3GB of "Small" images (less than or equal to 2048 pixels on both edges) - also free to store.
That works out to 143 GB of full-size, non-free data - exactly what is reported by the storage report!
So, I guess I could have saved myself a lot of bother by trusting Google and just forking out for more storage, but it still really should be easier to get this information from Google+ or Drive directly...