Opinions vary.
No. It is just your skewed opinion, which is different.
What mistake? The Mac uses DDR1, SAM460 uses DDR2. @ 400 rating DDR1 will outperform DDR2 at the same rating because of latency and overhead. If SAM460 can use memory faster than DDR2-400 then it will outperform the G4's DDR-400 memory. These are facts.
Facts end at "SAM460 uses DDR2". The rest are just your speculations. Especially stating facts starting the sentence from "if" is amusing.