Not another thread about this complete rumor. There is zero evidence of this, and in fact, considering that a) Apple invented FireWire, b) pretty much all PC chipsets have integrated FireWire and c) iMovie is useless without FireWire, it's pretty darned unlikely.
Just because you see i on the internet doesn't make it true.
As for CPU usage: yes, on Macs USB has higher CPU usage, but it has nothing to do with the "control chips". FireWire is not really any smarter than USB. It's purely a driver issue -- look at Windows, where the CPU load situation is reversed...