I think that a lot of people think about this in terms of ever more complex interactions and sophisticated algorithms. As I've said before, I think thinking in terms of algorithm is a bottleneck here. I think that thinking more in terms of basic forms is the right way to think about it. John Chowning said as much BTW.
So the advantage of more than 6 operators is largely in being able to construct more layers of those forms and then feeding the mixed results into other elements of the voice chain, e.g., filters, on a per voice level.
I feel that thinking in terms of Yamahaha's decades old approach is a severely limiting factor in FM synth design.
A friend of mine has one of those. I think it sounds cool, I'm trying to ditch hardware completely at the moment though. I can do some interesting stuff in Reakt0r, but, I don't have the time or energy to take it further. As much as I like Bazil1e, no really, I think that it's a fantastic synth, I find it irritating to do some of the things that I find interesting with FM. I know that Urs isn't really interested in taking it in a more technical direction, HS basically dismissed my interest in breakpoints. I don't find workarounds to be the same thing in terms of workflow and I would constantly run into annoyances trying to certain kinds of sounds. Basically, I want an 8 op FM8 with Urs' filter/oscillator/fx technology. Barring that, I want SuperBaz1lle, with double the oscillators and envelopes and proper complex envelopes, CPU load be damned.My 2 favorite FM synths are Bazille and the Elektron Digitone. Bazille is really something different and the Digitone is a simplified 4 operator FM synth that is pretty easy to make interesting sounds with.
It does sound cool. I'm staying disciplined though, no new hardware until all of the old and unused hardware is gone. Except for osmose, that doesn't count.The new Korg OPSix looks interesting as a hardware FM synth because it does stuff like Ring Mod, Filter FM and Wavefolding along with the using FM and has some decent filter models. I haven't tried it, but it also might be a pretty interface for hands on FM design.