@aBc Yeah, there's a thread (multiple threads?) from some years ago they got the 7448 in. I have to dig them up, it's been years I spent any time on MacRumors PPC. Like @indibil said, though, there's firmware patching involved. I forgot if it was the same patching also needed for any Mac OS 9 hardware with a 7448, and/or if there was also flashing of the mini's ROM. IIRC we did have to flash it, I think with a PowerMac10,1 (mini 1.25GHz / 1.42GHz models) ROM modified by dosdude1 so that the partial 7448 support in ROM would be not just partial, but complete. The side-effect of using the ROM, again IIRC, is that things like the serial number of the mini were also changed to dosdude1's mini. And, of course, PowerMac10,2 minis (1.33GHz / 1.5GHz) would be changed to PowerMac10,1's firmware, but that is actually a good thing, since our Mac OS 9 port for the mini works much, much better with the original minis, and that may perhaps improve them to work the same.
In any case, @indibil, don't give up the dream! Be patient with them. I almost "hired" JoyBed to do CPU resoldering for me years ago, as well, the only reason I didn't being the fact that I had to leave Europe along with nearly all my Mac hardware... But he's a cool guy. There's also an US-based guy that does what he does (forgot his name, he made a dual 7448 2.0GHz MDD daughtercard and a dual 7457 1.67GHz MDD one as well for me, as per my directions, which were never able to be shipped to me, and so at least the 7448 one silently went to Action Retro, of all people, instead...). To do it with regular MDD daughtercards, he used an tiny interposer board between the CPU and the socket so that the 7455/7457 processor pinnings they expect are remapped to the 7447/7448's pinnings (I made threads on MacRumors PPC about that, as well). Joybed is aware of his work, and years ago he showed interest in doing exactly that for MDDs and other Macs, as well.
By the way, regardless of processor, if you do break the 2.0GHz barrier somehow, now THAT would be a first for me. The US-interposer-board-guy tried many, many, many times to go beyond 2.0GHz with the 7448s for my MDDs, but he never seemed to get it to work, or to be stable. If it is possible, you will need some really good 7448s I suspect, by winning the silicon lottery big time. Perhaps what you are doing, which is to change the voltage on the minis, is a key to the answer of achieving that.
@Bolkonskij Transplant every PowerMac with 7448 at 2.0GHz!!!!!! I saw once a Reddit post of someone who did that on their System 7 PowerMac, and System 7 still boots!!!! Of course, it works like a super G3 since the AltiVec portion is ignored on anything before Mac OS 9, or to be pedantic anything before that "G4 Edition" (Sawtooth?) of Mac OS 8.6 that has partial AltiVec support.