Well, I doubt Sica just accidently made a good one
But they apprently did a good job both shaping the small horn that the tweeter is situated in, the way it integrates with the midbass cone, as well as the way the transision to the surround / outer ring is done. The surround hardly sticks out beyond the cone at all, as opposed to some alternatives, so not contributing much in terms of diffractions / problems. The tweeter is also surprisingly powerful with low compression and distortion even at elevated levels. At 110dB you're still at between 1-1.5% THD depending on the frequency, which is pretty insane. Actually the midrange gives up before the tweeter.
What can I say, it's just a damn good driver. I had the SBS.1 at Seas to do the measurements in their Klippel (those guys are the best by the way), they were somewhat miffed I wasn't using Seas of course, but I think even they are pretty impressed with this coax, though they'll never admit it.
What I don't understand in this design, is how you are summing the rear cone output with the front. The output from the rear of the cone is obviously out of phase with the front.
Just so members know what we are talking about, below is the radiation patter of a driver on a flat baffle, so a di-pole.
So there are nulls at the sides. The frequency of the cancellation will be lower the greater the width of the baffle. But obviously, what you are trying to do, is to make it seem as if the baffle is narrow, and control the forward response.
However, I don't understand how your design controls the inevitable cancellations between the forward radiation and the radiation from the rear via the openings.
It seems to me that unless the rear radiation is somehow reversed to be in phase with the front radiation, then the off axis polar response will be very lumpy.
I did consider this problem around fifty years ago and solved it. It was for a system I designed for a large venue where bands and music groups performed and was for sound reinforcement.
This was an active horn system. The bass sections used back loaded 15 JBL drivers in a bass horns the size of a small kiosks, with three foot horn mouths.
The top end was a two line sources of eight Jordan Watts drivers, these were mounted in the back box. There were throats down either side, and a short exponential flare either side. This solved the reflection problem, as the width of the back box was only the width of the drivers. The horn design, inverted the rear radiation, and the flare gave just the radiation pattern I wanted.
The system sounded absolutely marvelous, and reinforced bands and vocals beautifully. Because the system was so linear there was excellent forward gain from the mics.
One evening we had a really good Bohemian band up from ND. I was in Canada then. They sounded just wonderful. I made a stereo recording of the live mix, but sent mono to the two speaker stacks, built in either side of the stage.
The evening was so good, I issued an LP of the live mix, which in the upper Midwest became a really good seller.
I think something similar might give just the response and effect you are looking for.
Changing the subject. I had not heard of those SICA drivers previously. I see they are Italian. They seem to make a huge range of drivers, with parameters from low Q high FS to higher Q but still quite high FS. Many of the specs are usually seen in PA type pro drivers. I have not had nearly enough time yet to digest that "motherload".
They claim MC Audiotech of Florida as their US agents. But MC Audiotech, make no mention of being involved in any way with SICA or their drivers.
I do not see a source of those drivers in the US, but may be you can enlighten me further on that last point, and probably the other as well.