CK first may be better use of time
I'm a British medical student, who decided to go for USMLEs late on in medical school. Here, very little is taught on things like genetics and biochem, and often in other subjects, the detail is not quite the same, especially for minutiae. I'm going for CK first, as I'm currently in the middle of finals here, making CK revision sync very well. The things tested on Step 1 would not come up in finals, whereas there is little difference in material for step 2.
Following on from that, I anticipate that step 1 will cover many topics that are included in British post-grad exams, and so the revision will complement the exams at a later date.
I'm planning on taking CK in the next few months and will let you know how I did :sorry:. I'm currently working through USMLEworld, and getting OK scores.
In summary: for some IMGs who have graduated or come late to the party, doing the exams out of order may complement other work they're doing better.