Speculative processing is a feature of the G3 CPU's architecture where a G3 CPU "speculates" as to what the next instruction is likely to be, and spends free time processing on this speculation. If it's wrong when the next instruction comes, it just discards the incorrect results, but if it's right, performance is improved. Speculative processing results in a 2-3% speed increase, but the PCI PowerMac motherboards were built before speculative processing was developed on the G3, and therefore they do not support it.
When speculative processing is enabled in a PCI PowerMac, the CPU can accidentally alter data in volatile areas of memory. When those parts of memory are altered, crashes and erratic behavior can occur. Certain pieces of hardware, such as some Adaptec SCSI cards and certain high-end video systems, can stop the Mac from booting if speculative processing is enabled. Upgrade makers solve this problem by disabling speculative processing. Different upgrade makers disable it in different ways, some methods being more elegant and functional than others.
The Carrier 450 handles the issue of disabling speculative processing with a bit of software that XLR8 calls Virtual Firmware. Virtual Firmware loads extremely early in the boot cycle, before most other devices that could be affected by speculative processing, and disables it. The difficulty with Virtual Firmware comes when you add or change hardware. The Virtual Firmware may sometimes need to be replaced after such upgrades. Until you replace it, the machine may be unable to boot. There is a moderately easy work around included with the Carrier 450 instructions to restore your machine to working order if this does happen, but it can be frustrating if such a situation occurs.
The MAXpowr 466 handles the problem of speculative processing in a much more elegant and simple way. Newer Technology disables speculative processing in the hardware of the MAXpowr 466 itself. Therefore speculative processing is a non-issue with the MAXpowr 466. This is most likely why companies such as Media 100, a high-end video editing hardware company, solely recommends MAXpowr upgrades.
Because speculative processing is disabled on the MAXpowr 466, and speculative processing is the main cause of incompatibilities with G3 upgrades, it wins in the compatibility realm. If speculative processing is not an issue for your hardware and software, then the Carrier 450 is equally compatible.