I just performed an L3 cache and CPU swap on my 800Mhz Sonnet card. My 800Mhz came with a 7451 installed on it, so there were at least some of them made that way by Sonnet.
I started by swapping on 2MB of 250Mhz cache memory chips from a Quicksilver upgrade. The 2MB of cache showed up immediately with no other changes required. The Sonnet extension is what initializes the caches on these cards (this will be important later). Therefore, there aren't any resistor straps necessary to get it to recognize 2MB because the extension just does some address line probing and simply figures it out.
Next came installing a 1.25Ghz rated 7455B that I harvested off of an MDD CPU card. It booted at 800MHz without issue and also recognized the full 2MB of L3 Cache. Now that I had a working card, I decided to give it a spin in my Power Computing Power Center Pro 210, which actually runs at a 60MHz bus. By installing a crystal socket on the Sonnet card, I can now change the bus speed with a replacement oscillator. I had the PCP running stable with the bus at 68MHz with its stock 604e. However, with the 7455 installed, it refused to load video above a 60MHz bus.
Now for overclocking. Starting at the 28x multiplier and working my way down, I found that the highest multiplier that the machine would boot at was 24x. However, L3 cache would not be recognized at the resulting 1.44GHz (which, fair, that would run the cache at 360MHz). The fastest I could get the Mac booting with L3 cache enabled was at 21x for 1260MHz. The machine was definitely unstable at this 1.26GHz, likely because the cache was running at 315MHz. Thankfully, the XLR8 MachSpeed Control software allows on-the-fly editing of the L3_CLK bits in the L3CR register. As long as the cache is initialized (which the Sonnet extension can pull off at 1.26GHz) you can change the divisor. Dividing by 5 gets us 252MHz for the cache, which is just a hair over its rated speed. Once selecting this, I was able to get a (mostly) stable machine. Some software still acts funny and I'm not sure why, but it hangs in there for the most part. The XLR8 software also seems to set the cache speed to the last set divisor once its extension loads. This has me wondering if I can boot at 1.44GHz with cache enabled and run the cache at 288MHz or 240MHz. The CPU is already getting very hot with the tiny stock heatsink though, so I think I'm sticking with 1.26GHz. If the XLR8 trick worked, it would also stop working if your preferences files got trashed for any reason.
Finally, I wanted to see if I could boot System 7 on this bad boy. It did work with the stock 7451, even with the 2MB of L3 cache fully recognized. However, with the 7455 I could only get it to boot with extensions disabled (so no cache). After copying the XLR8 extensions and preferences from my Mac OS 9.1 partition, I could finally get it to load with extensions, but the key Sonnet extension will not load under any circumstances. This unfortunately means that both the L2 and L3 cache are disabled under 7.6.1 (the benchmark results show this in a rather unfortunate manner). It seems, at this time, the fastest CPU that System 7 will successfully boot with all caches enabled is a 7451.
I actually found a 7457 and it is on its way, but won't be here for 2-3 weeks. I plan on seeing if that works on this card once it arives. I also want to see how it acts in my Power Macintosh 7600 at some point. Maybe I'll have better luck with System 7 there?
Just a side note, I'll have this mean machine at VCFMW this weekend if anyone wants to come check it out.