I started setting my fore/aft by using the "hands off" method (being able to balance leaning forward with no hands). This occurs with the tip of my seat 8.5 cm behind the BB. But, at this point, when I get low, in the drops or aero bars, my speed & power drops like a rock. Moving back to 9 cm, I can get very low, without power loss. At 9.5 cm, my power drops significantly when I get into an upright position! (At least I know at what setback I'm the fastest!) But, does this sound right? Is it normal to have such a small range -- or, nearly no range, in this case -- in which I can efficiently pedal? Or, does this indicate a problem with my fit? (If I raise or lower my seat just 5 mm I also experience a significant loss in speed/power.) This also makes me wonder how time trialist get in to such a forward position. If I did that, apparently I'd end up going backwards! Any ideas?
It is not unusual to have a very small range of position which allows for your most efficient pedaling position.
It is a little unusual for it to be quite that drastic a change. Are you accounting for the change in saddle extension by adjusting your seat height as you move the saddle for and aft?
If not, I expect that it is more the extension length than your fore aft positioning which is affecting you.
By the way, 7-9cm behind the BB center is fairly far back when measuring from saddle tip. Most people are in a 2-5cm range, unless they are unusually tall or have unusually long femurs for their height. You may want a professional to take a look at your fit.
I hope that is helpful.