It most certainly is not, and I highly doubt you can find reputable benchmarks to support such a claim.
I think the 2:1 ratio is about right. The PowerPC is more efficient - it gets about as much work done in one clock cycle as a Pentium does in two cycles.
There are a couple of other factors to consider -
Modern Macs have Quartz Extreme, which greatly improves video performance. Windows won't have a similar hardware acceleration architecture until Longhorn is released, at the earliest.
Some Mac software uses the G4/G5 vector processing units which will easily outperform Pentium software running at much higher speeds.