So Danny, what does the big drop in resonance mean to us amateur speaker designers?
It can hit its resonance and you won't know it. It hits it real softly and doesn't ring.
Can we ignore the old rule of thumb about crossing at least 2x the resonance frequency?
No, you can cross a couple of hundred Hz off of it if you do it step enough.
If so, what is the lowest you would cross this?
I crossed them at 2kHz and it sounded really good.
Does the lower amount of ferro fluid significantly affect power handling?
It can if they are played hard over a long period. I have yet to see any of our tweeters die from too much power though.
Another question. Since you have heard a TON of tweeters out there, can you give us an idea of how this T3 tweeter compares to some of the megabuck soft domes - Scan Speak 9500 or the 9900 revelator series, or Seas Crescendo or Excel series?
I've done designs with all of those tweeters and found that the type of cap used can make just about as much difference as the difference between some of those tweeters.
I once compared our GR-T2 with a 9900 Scan Speak. I liked the Scan Speak better. It was clearly a little better across the board. But when I put an Axon poly cap on the Scan Speak and put a Sonicap on the GR-T2 it made it a pretty tough call.
How many other tweeters out there have been pre-tested and measured, come with their own response curve, and are sold in matched pairs for $29 a piece?

The Neo's are on a different level. They edge all of those out in my book.