It's pretty much certain that this "G5" is IBM's PowerPC 970 processor. That being said, with an actually good memory subsystem, this could potentially be a vector processing machine on par, if not surpassing even the P4 in SIMD (it has, theoretically, twice the SIMD throughput as the P4 per clock and we're talking 2 GHz vs the 3.2 GHz on the P4).
It's scalar processing capabilities aren't as impressive but still should be quite good. Perhaps not K8 good but still impressive nonetheless. Considering its PPC ISA and the 4-way superscalar design (or was it 5-way, I forget), it should be a very big contender.
Maybe now, Apple will finally put out a competitively performing machine worth the hefty price they charge.
"We are Microsoft, resistance is futile." - Bill Gates, 2015.