Well, the X5000 ought to outperform a Peg2. Even the low memory bandwidth figures you've quoted for the X5000 compare favorably to a G4.
And the integer performance would definitely be higher, floating point would not have AltiVec, but at almost twice the speed that would also be better.
Then there is a video slot differences.
Even limited to four lanes a Gen2 PCI-E connection is going to provide better bandwidth than the modified PCI slot that the Peg2 uses for AGP.
And the X5000's PCI-E slot can handle modern Radeon HD video cards which the Peg2 can not.
In fact, TBMK the Peg2 tops out with the Radeon 9800 as it cannot use cards intended for 4x/8x AGP slots (it provides too high a voltage to the AGP slot).
So while MorphOS support AGP variants of the R400 and R500 video cards (and may eventually provide support for R600 and R700 AGP cards), these can never be used in a Pegasos.

So, while I have a strong affinity for the Peg2 as a native MorphOS platform, my G4 system is a PowerMac MDD.
It's faster, has an AGP slot I can force 8x AGP card to work in (by masking two pins), and they are dirt cheap (as well as commonly available).

I agree with you that the Quad G5 would make a good MorphOS platform, but there has been no commitment to port to that system.
And a port is being prepared for the X5000.
So, while an X5000 isn't a G5, it is better than a G4, it has PCI-E expansion, and it's available new.

And the price isn't that unreasonable, considering the price of a new Mac or the Lenovo laptops I buy.

[ Edited by Jim 14.09.2017 - 10:56 ]