A good 68K can outperform them, hands down - on a clock per clock comparison.
But what do you really want?
If you have an A500 or A600 or A1200 then you ideally look
* for a good CPU card that does not run to hot in your Amiga case. Am I right?
* and you want a card which works well with your Amiga power supply,
so that you do not need to use a 100 Watt PC power supply for it.
This means for many chips your AMIGA does not have good enough cooling and not good enough power supply.
An FPGA based 68K would be happy with this cooling requirements.
And performance wise it can beat PowerUP PPC cards
and does beat hands down stuff like 240 Mhz Coldfire or low clocked ARMs.
So with the low end chips a good 68K has no problem to compete.
Do you want to run 68K code?
Then this means you need to run 68K in emulation on your ARM.
In this case a good 68K can easily compete and beat the absolute high end ARM chips.
I have an A500 (stock, since I sold my GVP A530), an A600 (w/ACA630, A604, Subway USB, & wireless NIC) and an A1200 (w/Blizzard 060 & SCSI, Subway USB & wireless NIC), as well as many other Classic Amiga models in my collection and both flavors of PPC NG systems, using an X1000 for AmigaOS4.x and several G4 & G5 Mac's for MorphOS. But even with all of these options available to me, I was quite saddened when the Natami project became stalled, or stopped (I don't know if it is cancelled, or just on hold).
Do you know if there is any work still going on behind the scenes on the Natami project with the few developers who actually were lucky enough to get a Natami MX motherboard? Is there any chance that it will get restarted, revived, or taken over by other developers who are working with FPGA's?
Or is it more likely that something different, but similar might be produced by the people like you who are currently working on FPGA systems to run AmigaOS and software?
Even though I like all of the variety and capabilities of all the Amiga Inspired platforms, I still would prefer the Natami approach, if we could get a working SAGA, or AAA, or what ever you want to call an improved graphics system. It does not have to be "State of the Art", or compete with any modern systems, it just has to be one order of magnitude better than the last Commodore AGA system, plus it needs to have some modern connectivity ports and drivers. Some programming tools would be a big help too, or just update the old 68k programming tools to make it easy to create new software for this imaginary new updated Amiga system.
I would love to read that people are still working on making the Natami, or a similar system a reality, and that something will be finished within the next few months.