porting the QEMU CPU emulation to DOSBox would be very attractive since it's faster.

That's exactly the point: porting that requires changes (like memory access)
that will result in something slower, so there's no point in doing that.

Maybe, if that's where all the speed gain comes from, or that may not be everything, also, nothing stopping from replacing memory handling as well.

You can't replace that. But feel free to speed up that in dosbox 😀 (128bit transfers left aside)

Actually, several of the patches I had made included the addition of 64 bit memory and cache accesses, along with the accompanying use in decoding and blitting, but those changes were never incorporated into DOSBox.

They only helped for some artificial tests like profiling the non-recompiled part. So they are of no use to me as already noted.