The main benefit of the PPC970/G5 is the improved CPU interface to the rest of the system. To cripple it in an "upgrade" by latching it to the same slow bus that the G4's currently use is like throwing money away.
In that case, the PPC970/G5 may perform WORSE than the G4's of the same or lower speeds. Why? The G4's are more optimized to get the most out of the limitations of their current interface, 'ell, check out their cache arrangements. The PPC970/G5 is designed from the ground up (originally as POWER4) to expect a certain minimum bandwidth feeding the CPU. This assumption is even higher than the POWER4 since IBM's going to likely be using the same processor interconnect technology it co-developed with Apple for their PowerMac G5.
The only thing you'll get with the PPC970/G5 over a current G4 is native crunching of 64-bit integers (oohhhh) and a 64-bit flat memory model. For the A1's 2 DIMM slots (it is 2, isn't it?). I can't imagine people putting in two 4 GB DIMMs to bump their G5-upgraded A1 to 8 GB. That would be on the order of insanity, far above stupidity--if the A1's motherboard/firmware/chipset even supported that.
So what do you end up with? A motherboard swap to get the most out of a G5, ok, that adds to the expense and complexity. A CPU-only replacement which cripples the CPU and reduces the benefits of a native 64-bit CPU. Or a totally new system, A2 or whatever.
I don't think there will be any G5 upgrades for the old PowerMac G4 systems and if there are, I think they will not perform nearly as well as people would expect them to.
But that's just my opinion.