Steve,
Like Terry, I too question the real-world value of a super-fast toolchange time. I can't imagine that it would save more than a few minutes per day at the cost of significantly increased complexity, and opportunity for disaster, due to higher speeds, and generally higher precision required. I think you can quickly get to the point where doubling speed will take 10X the time and 2X the cost, to make it as reliable as a slower, simpler machine.
Re: the speed of a Geneva, this, too, I think is a bit of a red herring. Perhaps a high-speed Geneva will always be a bit slower than a max-performance direct-driven mechanism, but I think the overall difference in toolchange time would still be small. I'm running mine VERY conservatively, and it runs under a second per tool pocket. So, even with a 24-tool carousel, max seek time would be 12 seconds. I'm sure I could easily double that. Beyond that point, you have to start worrying about the tools flying out as the thing moves. The biggest job I ever did only used about 12 tools, and that one took hours to run, so an additional 3 minutes in seek time would be lost in the noise.
Certainly doing a variable-tool-size chain-driven machine is quite do-able, but, again, at considerable cost in terms of complexity. You'd have to not only program each tool length, but now the "pocket" width as well. And, setting up a job will mean a fair amount of disassembly/re-assembly to move the tools to their required positions. Or, have pre-configured slots of different sizes, but then you have to deal with mapping tool numbers so each tool lands in an appropriately-sized pocket. My biggest tool is a 4" face mill, so I just sized all the pockets accordingly.
Regards,
Ray L.