I think since you are asking specifically about v9, which is still unannounced, it's going to be very hard fro anyone to answer your questions, aside from ADS...
All I can tell you is my personal feedback having used all of the available rendering engines for FormZ.
Thea, like you, I bought Thea and really liked where it was headed. However, until there is more demand, it doesn't look like it's going to be updated. Since being acquired by Altair, Solid Iris is no longer calling the shots and many of their other plugins are no longer being currently developed. I hope this changes in the future, since it is a great, high value option.
Maxwell, I've used several demos, off and on over the years testing the possibility of purchase, and the speed is just too painfully slow to justify (On a 12c/24t Mac Pro). That coupled with the high costs and the fact that we wouldn't be able to share our models to others unless they also incurred the high cost of the renderer. It's definitely powerful and can deliver great results, but in 2017, it's very slow, there are other pure CPU renders (Arnold) that are much faster, just not available for FormZ. They brought in a GPU rendering option this year, but to be honest, it's still very much in testing mode, and many features do not work, even with very powerful new GPUs. And it's still limited to 1 GPU which largely defeats the purpose of a GPU renderer, as you want to be able to stack as many as you can in the same workstation to avoid additional rendering licenses like you would need with another rendering node. Maxwell feels like a relic of the old days of rendering, not unlike RenderZone, certainly more powerful, but just pricey and cumbersome compared to the new options.
RenderZone, seriously it's been a superb integration in FormZ. The flexibility is amazing and very forgiving to new users. It's just old now. Once you have used Progressive rendering and emitter materials of other engines, you'll NEVER want to use RenderZone again. I also think that Shaded Full has largely filled in this gap and would much rather see that advanced than any more time or money be spent on RenderZone.
Shaded Full, I added this one because once you start using it for concepts, it's addictive. The speed when using a modern GPU is incredible. The shortfall is that it's OpenGL based, which means it's crippled on the Macs compared to Windows. This is Apple's fault, plain and simple for not updating to the newest OpenGL as they are released. That said, if there were a way for it to continue to be improved, I'd love to see it! Particularly emitter materials and saving out fly through animations! One las thing, Shaded materials translate the best when moving to Thea or Vray, without having to recreate them 100% of the time unlike trying to use RenderZone materials with the other engines.
VRay, I think every FormZ user should be excited about this one. It's an old industry standard renderer, but it's been constantly updated and modernized, and in these newest implementations is MUCH simpler to use than the old 3dStudio versions. The Vray FormZ plugin is even getting the latest Hybrid Rendering Engine from Vray 3.6! That means CPU + GPU rendering power, not unlike Thea's Presto. VRAY on a modern workstation, say a 16c/32t AMD Threadripper 1950x and a couple of Nvidia GTX 1070s, and you have an incredibly fast Vray rendering machine for much much less that was previously possible just a few months ago! The new Denoiser also helps drop rendering times dramatically! I think this plugin is getting very close to a public release and I'm looking forward to it being widely adopted. Since it's an industry standard, sharing Vray rendered FormZ files with other applications is now much easier using the .vrscene export. - https://labs.chaosgr...n-applications/