I don't know about 55 but 50's work well in my k-hornet and with lil'gun there is only about 100-150 fps difference, but you need to use the right bullet. The two heads I have tried are the 50 sierra blitz and 50 Hornady spsx, I just tried the spsx and they seem softer, used it on some finishers on coyotes last week had no exits so there had to be decent expansion.
You should be able to get 2700 fps with lil'gun and a 50 grain head I don't know th max speed for the seirra head but if memory serves the hornady is 3300fps.
McLernon a while back I think plain ol' bill posted that after talking to the techs , the 40 gr Sierra is the softest he was noting that most of the other 40's are designed to handle 4000 fps so jackets are pretty tough.