You have an impressive system and I'm sure that you enjoy listening to it.

You are on target that I do not recommend tapering a tweeter line as you will have few benefits. Tapering a line of woofers would permit you to adjust for room effects (or what I call sound bloom). Tweeters shouldn't need to be tapered as they are likely operating well into the near field region so floor and ceiling reflections aren't an issue.

On the increase of the number of tweeters in your current system: I'm not sure that I have enough knowledge about the details of your current system to make a knowledgeable comment. I will comment that if your current crossover frequency between the small tweeters and the long planar driver occurs at a relatively high frequency (for example, in the 15,000 Hz area), then you may not yield significant improvement in overall sound quality by going to 12 tweeters. My thinking is that there isn't much program material in this upper half octave (between 15,000 to 20,000 Hz) so you may not hear enough of a difference to warrant the change.

Jim