Oh, I see...
Thread 0 reads pixels: 0, 1, 2, 3, ... 31
Thread 1 reads pixels: 32,33,34, ... 63
Thread 2 reads pixels: 64,65,66, ... 95

But memory access pattern for wavefront is:
step1: th0: 0, th1: 32, th2: 64, 96, 128, 160, 192, 224, 256, ...
step2: th0: 1, th1: 33, th2: 65, 97, 129, 161, 193, 225, 257, ...
...

I'm not sure that it is a good pattern, so some investigation is required. Especially for C3 data.