It seems like the main theme of this thread is that distance is determined by disc speed rating.
To say that you need something between a 5 and a 9 may not be completely accurate.
Doesn't speed just indicate the level required to attain optimum performance?
You've already said that your speed 7 Eagle flies farther than your speed 9 Firebird, yet you are still looking for something in the 6-7 range.
Instead, maybe you need a 4-5 speed disc with more glide.
Or maybe a 9-10 with more HSS?
Why is it you don't like to throw between 60-80% power?
Seems like if you have a disc that you are comfortable and familiar with, it might just be better to take a little off.
That's just me.
Do whatever you like.