There's nothing wrong wth the mini. But the 7447 and teh Apple northbridge aren't really a new chips. While the e600 ALU is nice and better than most other ppc ALUs, the 7447 uses a very old FSB interface and a dedicated northbridge.
The Sam's 460 while not having a fast/high clocked ALU has a better FSB though (no wonder, the chip is low end but rather modern).
Throughput is a major key factor for the overall performance. It is also the reason why an 8610 board would have outperformed all other G4 ppc systems on most operations (not pure numbercrunchuing like OGR though) - an e600 core with a fast FSB.
Anyway, while the Sam460 figures show that this Sam will proabably offer at least a power nice enough for much it is also clear, that it is stil an entire class below the mini in terms of computing power.