Zaphs is a great resource for driver testing, though he doesnt currently have the off-axis response up there. Accurate to boot too. And he has a speal on offaxis response to, quoted below
Quote
The importance of top octave response is highly overrated. It has absolutely nothing to do with sound quality, and everything to do with flexibility of placement. Simply, all it means is that you might want to toe larger drivers inwards. I say "might" because the usefulness of a response that's flat out to 20kHz is also overrated. While a peak at 10kHz is easily heard, (tape hiss anyone?) a few dB down at 10kHz is barely audible, and in some cases preferable. At the very top end, 15dB down at 20kHz is barely audible if at all. This is probably why many people don't mind tweeters like the Dayton RS28 or the many of the Morels, such as the MDT20. Dips in response at the extremes of audibility simply aren't very audible, even for those who still have clinically proven hearing out to 20kHz. (I'm good to about 17kHz in my old age)
www.zaphaudio.com/offaxis.html
that being said, he did have this to say on the Aircirc "off axis response is closer to normal for a 1" dome". With that statement alone, I'm fairly convinced that the 6600 isnt anything special in regards to rolloff.
And that link that Luke provided has the offaxis responses for the Hiq all the way down to 75-odd degrees.