Can't fight physics!
Going a 2.5 way with 10's makes sense but as always, the center-to-center distance rules the game. My first "fresh sheet of paper" speaker build back in 2011 was a horizontal center with four 5.25" woofers, a 30mm WaveCor tweeter and it was configured as a 2.5 way crossing at 550Hz and 1,800 Hz. It worked well with my L/R at the time--as far as I could get it. I "nested" the large dome tweeter up at the edge of the baffle and the tweeter/mid-woofer frames were touching to get the C2C distance at one octave. My only major complaint with it was efficiency, not bad at around 91dB but I needed more.
These days I use an 88 Special as a center because it would fit my heavily modified furnture to contain the thing. Really liked the 1099 but it was larger than the frame of my furniture no matter how I sliced it.
The HTM 1010 looks interesting and being a 2.5 way--optimised as far as you can go with that design. My issue is simply this, it is about the same size as the 1099 so....why? The HTM series needs a horizontal center so take the 16 ohm HTM 10 inchers, the HTM compression driver and the 1099 mids to make a "HTM 1099 Center". Sure, it won't "match" because of the mids but a 70 x 70 degree horn won't "match" the SEOS 90 x 45 horn anyway--so compromise is inherent in the design. At least the woofers, compression driver and horn will "match". The only real positive I see with the 1010 over the 1099 is it will be slightly smaller--but not by much because it runs the ports on the sides (I would of put the ports above/below the small horn to cut the width) Running 8's instead of 10's still has the center-to-center issue but at least it is considerably smaller than the 1010/1099. If you spent the bucks to get 16 ohm 8 inch HTM drivers, then it would be easy to make an "HTM 898 Center" or use the HTM compression driver in the 10" SEOS horn, the two 5" mids under it and the 8's flanking it on either side. To make the bezel as compact as possible, use the single ports of the Fusion 8's and place them under the 8's to cut the width because ports are not taking up space on the sides. This will cut the width down to around 30" wide or slightly smaller but still have at least 97dB or higher 1w/1m of efficiency. I pondered why the HTM 8 was delayed for a year after the design was finished and the pictures of it leaked on the net--I assumed because it was waiting for a center channel to "match".
I seriously thought of going with HTM 8's this past spring as LCR and rebuild my Fusion 10's as high powered rear surrounds--glad I scrapped that idea and went with the 88 Special to finish my LCR. Just from a design standpoint, I would prefer the 1099 but went with the 88 Special for width issues. Now that the 1010 is beta, I'm glad I went that way because I need a very good center with rather wide dispersion (better than +/- 15 degrees) It is good to have options so the HTM is for screen use, the 99 series is for horizontal centers and people that prefer 3-ways with the remaining Fusion 8 and 88 Special probably marked for elimination in the future. I can see them going away when the HTM 8 arrives.
I do like the 70 x 70 horn for surrounds--the HTM-6 with that horn would fit Dolby's dispersion demands for surrounds (54 degreex X 54 degrees when aimed at MLP) The "HTM 6S" for surround with that wider dispersion horn would work well for surround speakers. Maybe throw together an HTM-8S with the same horn for better flexibility for surround speakers due to running the smaller horn. Once a person starts hanging speakers everywhere, the size and flexibility to make them fit becomes far more critical. Mixing that 70x70 with the HTM 6 neo woofer would make a very small, light weight speaker for Atmos and give wider dispersion than the typical SEOS horn allows. Sounds like a win to me!
Thanks for the update, hope your rest did the trick and looking forward to the polar charts/graphs for the upcoming 10/12/15" coaxials rolling out next month.