Should my fore/aft range be so narrow?
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.