>> I don't think a 1.5 GHz Cortex-A9 would be a match for the
>> 1.5 GHz G4 in my Mac mini.

> My personal benchmark for this would be 1080p video which
> even a slower A9 should be able to do.

Cortex-A9 at 1.5 Ghz or slower is definitely not able to do that. What is able to do that in many ARM chips is an auxiliary video decoding core that happens to reside in the same chip as the Cortex-A9. Utilizing such video decoding core under MorphOS would of course require a special MorphOS driver to be written for that particular core, not different from when you'd put a card with such video decoding chip into your PowerMac G4.