After doing some research and checking the manufacturers catalogues for cone drivers, up to 5" diameter, this is my short list.

There doesn't seem to be a standard for what defines a drivers size, its all very approximate, so I have included the Frame Diameter and the Surround Diameter (measured peak to peak) to show how they compare. The bottom pictures are to give a sense of scale. The C-Quenze is the largest and about as big as I can squeeze into my intended cabinet, by the time it has a closed chamber around it. All these manufacturers make a slightly larger mid driver within their range, in some cases, with a better spec, but these are too big for the space I have available.

My initial thoughts on these drivers, is that they are all of high quality.

After some consideration, I think the Satori, which is approx half the price of the other two, may not have enough power handling (30w) and may not match well with my big Scanspeak Woofer, which can handle about 300w and has massive xmax capability.

The Illuminator has a good spec/response and much larger power handling.

The C-Quenze does not measure quite as well as the others, but by no means measures poorly, subjectively it is preferred by many and described as natural in tone with great dynamics. One Diy enthusiast I came across, who owns both the Illuminator and the C-Quenze, prefers the sound of the C-Quenze. Troels Grevesen described the C-Quenze 18H (next size up) as "the best 6" midrange I've had". On the 15H52 specifically, in one of his projects he said "What strikes the most is the amount of detail from the 15H52 midrange. It's .... whauuu! The power handling, the dynamic headroom, its dispersion, its presence quality, etc. In one way it's a demanding midrange, in another it's not. No matter what you do to it, it sounds great". This from a man who has, from experience, a very high regard for both the Illuminator and Satori offerings. The Mineral filled Polyprop cone used on the C-Quenze is not as flexible as the natural/cheaper Polyprop used in low end woofers and I have found many people who swear by its subjective sound quality. In addition, I have owned several pairs of Tannoy M20 Mercury speakers, these have a Polyprop Midwoofer and I love the way they handle midrange, acoustic instruments and voices. This and the many positive DIY build comments, is swaying my judgment towards the C-Quenze at present.

Jan 2018 -  I have shelved this project.
After re-listening to the stock passive Yamaha NS-1000M I just cant get away from the fact that I like so much of what they do. I have made big inroads into establishing how to remedy what I don't like, so will be concentrating on the Yamaha project for now.