Exotic materials, other than maybe SiGe use fabrication techniques less advanced than Si, and the transistor counts are much less.

The department of defense funded an SBIR grant in the late 1990s to produce an InP based microprocessor, given the limits of the time it would have been closer to a 6502 than a Pentium. There has not been word of such a thing since which leads me to conclude that the topic is classified.

The worst limitation a 6502-era chip has is that it has no instruction cache so instruction reads are fighting with data for memory bandwidth. You might even consider a Harvard architecture where the instructions go on a different bus. Without an I-Cache there is no point in pipelining, but there is a lot of pressure to implement CISCy instructions such as the string copy operation from the 8086 line.

The other issue is that there is no DRAM replacement with exotic materials, and all the difficulties with interconnect latency get a lot worse than they already are. It's more clear how to make SRAM, so having somewhere between 64K to 1Mbytes of SRAM on die seems likely for an exotic material CPU.

Of course, armchair CPU designers are more likely to make progress with transition triggered architectures and FPGAs in 2020.