I have tried old style Avid shorty 6, new style shorty 6 and Kore cantis as in the picture above with std pads, kools stops and swissstop pads with differing straddle lengths and various toe in combinations. None had adequate braking or lack of squealing.
I now run tektro mini vee's RX5's and they work work well with shimano pads and cost about £20, they do run close to the rim which could cause a problem in muddy CX conditions. I can't speak about Avid Ultimates as I haven't tried them.