"Subject: 2006 1,1 MacPro CPU Upgrade, no kernel panics (in Lion or 10.6.8)
Like many others I did the eBay upgrade of my 2006 Mac Pro. I've not had a single issue. The upgrade went off without a hitch, and my computer routinely runs for 30+ days between updates first on 10.6.8 and then on 10.7.1. I didn't bother installing AboutThisMac under 10.7.1, it's running completely stock. (AboutThisMac not compatible w/Lion per a reader. It was used to correctly ID quad-core CPUs in mac pro 1,1's with earlier OS's.)
   The only thing I can think of is that unlike many others the 2x quad 2.66 had the last stepping (revision) available. (Maybe corrected some errata?) I also purchased an Apple branded ATI 5770 at the time of the upgrade, have been very pleased with the results. As usual, many thanks for the site, I wouldn't have thought to do this upgrade without xlr8yourmac.com
(he later wrote)
I read more on the subject on your site, so I went to gather some information for you. I went from the stock 2x dual 2.66 (5150?) to 2x Quad Xeon 5355 2.66GHz SLAEG (stepping code -- the last revision of the 5355)

Boot ROM Version: MP11.005C.B08
SMC Version (system): 1.7f10

I've never had any problems in 10.6.8 or 10.7.x, and it's all stock other than the CPUs and the upgraded graphics card.
-Dave"

For those that had Kernel Panics (and said the EFI 2,1 update helped), there was a theory that perhaps the 2,1 rom (the first quad-core Mac Pro EFI) may have had microcode fixes for errata in some Quad-Core CPUs/steppings - or some other improvements (being a later version I'd expect some minor ones at least).
  But those with KPs said it only appeared after updating to Lion (or less common, 10.6.8). So as mentioned in the first post on this subject (last Monday), I had asked if the usual Troubleshooting tips were tried (i.e. reset nvram/pram, smc, clear caches, check for corrupted files/prefs, reinstall update via D/L updater, check for incompatible ext/sw, etc.) - that did not help he said, but the 2,1 rom update seems to have. (He was not alone in reporting the 2,1 rom helped.) However a reader on Tuesday said a pram zap (reset) alone seems to have solved his KP's with a Quad-core (5355's) upgraded 2006 Mac Pro w/Lion.
 (Resetting the pram/nvram is an old tip going back decades (used it on my 1984 Mac IIRC) and is still listed in OS X Help. It clears some settings however and may lose the startup disk setting (if completely/properly cleared), so use Option key at boot if need be. OS X Help has a note on that with RAID volumes.)
 

"Thanks to all of the information here, I was encouraged to upgrade my son's Mac Mini 1,1 1.66gHz model.
After acquiring a T7200 2.0gHz CPU from a notebook pull, we proceeded with the surgery. All went very well, I was even able to remove the plastic heatsink pins by using an electronics needle nose pliers and very carefully releasing them.
Removing the flat cable connection to the drive board is not a good idea, in my opinion; I removed it, and reassembly was a a bit tricky; it could have very easily been ruined in the process, I definitely won't be doing that again.

The results are great so far. The Geekbench score improved from 2117 to 2841! CPU temps seem like they're in the same general operating range as before. Next I'll have to get an SSD drive...
-Mike S.
Texas"

The Mini CPU Upgrades page also has several reports on OS X Lion on C2D upgraded Minis.
 

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