I tried a SRAM 12-28 cassette on my bike when I was running 105 5600. It never shifted right and it was noisy. Spent hours trying to adjust it. Swapped back to my 11-28 6700 ultegra cassette and everything was fine again. I've heard they're 100% compatible... but it didn't work for me.

I've also tried to do a hybrid cassette to get a shimano 12-28 with a 16T. took the lower section of my 12-25 (105 5600) and the upper part of my 11-28 (ultegra 6700) to make a 12-28 (12, 13, 14, 15, 16, 17, 19, 21, 24, 28). That shifted almost perfectly.