Motorola got into the PowerPC project because IBM had persuaded Apple to switch from 680x0 to a new chip based on their POWER architecture. Apple invited Motorola to join. Apple knew that Motorola had more experience in making single chip microprocessors, but also having two sources gave them more bargaining power.
Sure - as said, RISC ("risk") was a fashion statement back then, and people believed that it would be a beneficial architecture. In some sense, this is true, but I would believe that history tells now something different.
I'm sure there are examples of 1 68060 instruction needing to be expanded to 3 PPC instructions. But the question I would ask is how often those types of expansions occur & whether it's mitigated by any situations where the opposite occurs and whether the PPC can run multiple instructions more often than the 68060.
At the expense of higher memory throughput, sure. So let's see some facts. If we believe Wikipedia, then the 68060 has 110Mips at 75Mhz, the PPC601 157 Mips at 80Mhz, which is 1.46Mips/MHz for the 68K and 1.96 for the PPC, thus indeed better.
This improvement is mostly, as I believe, because the IPC of the PPC is higher, due to the simpler instruction set. IPC of the 68060 is only 1.3, of the PPC 1.9. Of course, the PPC is also a later design, so it's a bit apples vs. oranges, as most of these tests.
At least this source:
http://www.microapl.co.uk/Porting/ColdFire/cf_68k_diffs.html also critically remarks the lower code density of the PPC, which seems quite natural given the simplicity of the instruction set and the design. Unfortunately, no hard numbers there on particular algorithms.
I'm reasonably confident that if someone put the same effort in then they could create a PPC that has a similar real world performance to a 100mhz 68060.
Well, maybe, I'm not a technical expert in this field, though one should note that such a design would still be far behind existing PPCs available on the market, so the attempt would be a bit pointless.
What would make sense is for there to be a board that allows you to fit an FPGA and an off the shelf PowerPC, so that people don't have to choose between vampire or a phase 5 PPC.
Again, I consider this somewhat pointless given the rather small software library for PPC on the Amiga - or possibly - the typical "applications" Amiga has found today. That's of course a completely different argument.
It's in my eyes mainly a retro system - if you want to make it fast by a modern CPU, one would pick an intel design and RTG graphics and not PPC and custom chip graphics. Wait, that's called a PC, right? (-: