Don't know how stiff different 56's would be, the only real way is to rate test them, the next best is to calculate the rate, after that we're just throwing out educated guesses. Practical experience seems to be a good way to go, finding people that use them is a good idea.
I think Jon Snow ramped his k5 on a 30 degree and did something in the low 600's with stock rears, they flex pretty well as is but you can get more capacity and still have decent flex from a longer spring.