We gain some FPS through instancing but it's nowhere near double so not sure where you picked that up John. Then with the new lighting and TE, avatars etc it runs at around the same fps as it did previously just with much higher detail levels.
I searched and tried to find the original quote for the frame rates being nearly doubled with SpeedTree GPU instancing, but all I can find is some follow-up discussion about it, wanting to know if SpeedTree instancing was going to make it into Unity 5.4. Maybe the original thread got deleted, but I do remember being surprised when I originally read it. I then read that SpeedTree instancing was being pushed out to Unity 5.5.
I'll have to reel in my expectations a bit, but at least you do see a performance gain from instancing, enough so that you're seeing the same frame rates as before, with all the new features and graphical enhancements. I can certainly live with that as I tend to run in the 50 to 55 FPS range currently.