The subject tells all...

My understanding is that stock 1.25 GHz CPUs are easier to overclock than 1.42 because the latter is already running on a higher core voltage. So it is relatively easy to get 1.25 clocked 1.5 (stable), while the 1.42 will not be stable at past 1.58 GHz.

But then 1.42 ones have 2MB L3 cache vs 1MB on those clocked at 1.25 GHz. Under what conditions would that be an advantage?

Then how those compare against the top end Mac Mini clocked at 1.5 GHz? For example, my Mac Mini can play 720p video but CPU is maxed out at 96+ percent.

`on morphOS the problem is the lack of graphics memory, I'd rather downclock my 1.5 to 1.33(ish) to quieten it down*, than have a 32mb Graphics card at 1.6mHz to be honest.

What I'm saying is I don't think you'll notice much difference in actual performance, but hey, some people can put AMiga1200 into A600 cases, so if it makes you happy...

*My MacMini 1.5gHz seems to have a hot CPU, the fans always kicking in, it's all clean, fans moves fine, worse on MacOS than MorphOS etc etc. Other people say their 1.5s are fine even overclocked.

Sorry, I have caused confusion - the intent was to ask about comparison of CPUs in FW800 model, not Mac Mini.

I wasn't aware that the 1.42 had that larger 2MB cache. I was considering buying a 1.33Ghz Xserve processor to get that feature.
And to answer your question, while not extremely significant, a processor with a larger cache would perform better (at similar speeds).
I wouldn't overclock a 1.25 as high as you've mentioned without a copper heatsink from a 1.42Ghz processor.
And I've seen 1.42s overclocked slightly higher than you've mentioned.

My 1.25 is OCed to 1.5 just fine, needed no core voltage upping or copper heatsink, but if I find one I may OC to 1.67 but that requires voltage upgrade. i have seen dual 1.42 running 1.8 before, but w/ max fan speed.

That's remarkable. I've got one copper heatsink I'm tracking on Ebay if you'd like a reference to it I can send it to you (personally I think its a tad overpriced).
Thanks for mentioning the 1.42 overclocks. I've seen decent overclocks on that processor before, but nothing like that (then again, if the 7447 and 7448 will clock that high why not a 7455).