Sure it sounds impressive being (x) times faster than the original 68000 but isn't the Amiga about multitasking and not about how many rpm's the cpu will do? Amiga and the PC have very different philosophies to what is measurable performance so maybe this kind of benchmark would be more appropriate for PC's?
CPU horsepower is CPU horsepower, though. Neither the x86 nor the 68k are truly multi-tasking by nature, so any multitasking OS is really just dividing up CPU time among the different tasks. More CPU horsepower = more cycles to go around in a given period of time. Of course, most Amiga applications probably aren't going to be demanding full power, but having more cycles to go around means that the tasks that are high-performance will be taking less of a bite out of the low-performance tasks, so everything benefits.
If the processor has support for virtualization of instructions (unimplemented instruction exception vector like in some processors) I guess you could potentially address that in software.
The 68k has had an illegal-instruction exception since the get-go. The bigger issue would be instructions that might be legal but operate differently (i.e. if they set some of the flags slightly differently than on the 68k.) Those would be legal and thus not get trapped, but still cause unexpected results in old software.