If by efficent, you mean by ways of instruction count (# of instructions the processor must handle to get something done), then yes, p4s are inneficent compared to AMD XP processors (Not sure about mac stuff). That is why an AMD XP 1800+ actually runs at 1.533 mhz, but will preform on par with a p4 1.8ghz.
When comparing an Intel processor's 20 step pipeline to a G4's 7 step pipeline, you have to take into consideration the fact that a G4 is a risc processor, so every operation no matter how simple, or complex, will take 7 steps. Whereas Intel's 20 step pipeline is a sisc processor (not sure about the abbreviation), but it means that not every processor instruction takes 20 steps, most take far less.
I don't know about p5s or what they're doing to increase efficency, but Intel's next mobile processor is the bainas (Or something) processor. It is going to be a fairly low speed (<2ghz I think), but it will have VERY low power consumption, and probably be more efficent, to help out the power-hungry users deal with the low mhz.