What's the single biggest and *stable* upgrade you've seen? I had a PowerMac 7500, which came with 100 MHZ 601 CPU. I upgraded it to a 400 MHZ G3 and it worked very well. I assume a 400 MHZ G4 would have been great too.
So, that's 4x MHZ, and 2-step processor (601, 604, G3).
That whole line, from the 7500 - 9600 was good for upgrading. I think it was basically the "Pro" line of its day.
Indeed the 7300-9600 were the "Pro" computers of their day. They were used extensively by creative professionals and mathematics modelers until it was clear Apple did not have a viable path for a modern Mac OS (Mac OS X was not a path, but an entirely new OS, so…) The 7200-8600 were all the same motherboard, basically. The 9500/9600 had a very similar motherboard but with a second PCI controller for more slots.
There is a hard speed limit for most vintage processors based on the bus speed. Most G3 could only run at 10x the bus speed and know that the 1st gen PCI PowerMacs only had 50MHz buses. There were late-model G3 accelerators that broke this barrier (I don't know how), but they were rare and expensive and not worth it compared to G4 upgrades. The G3 was based on the 603e, so it runs very cool, especially compared to the 604s most people were upgrading. The G4 was basically a G3 with the FPU from the 604e added along with the AltiVec unit (doesn't run as cool). The 604e, and subsequently G4, was a BEAST in FPU performance in its day.
Assuming there would be enough interest, the best upgrade path for vintage Macs would be FPGA. There are several FPGA boards with PowerPC cores. Right now, most vintage computer enthusiasts are focused on 68k AIO Macs. In fact, there's already ROM upgrades for Mac II-era machines to make them 32-bit clean and add ROM disks for booting.
