As long as you remember that the K4 wasn't designed with Ni200 builds in mind...
I know some folks have had great success with them, but it seems like those are isolated cases.
I have had absolutely no problem at all with the K4 and Ni200. Back a bit I expressed the same hesitancy as others, likely in the
dna 40, NR, Nickel Builds
Thread). Jumped in, with the experience building Ni200 I already had, and built without any preconceived notions.
@qorax: I would jump immediately to 3 mm. And as an initial test use KGD. (I know how much you love rayon and have perfected your cut away wick technique

.)
(1) Cut a piece of KGD about 2.5 mm thick,
(2) as usual remove the two stiff facings,
(3) twist the end and insert.
(4) If you feel there's too much wick, pinch a bit off.
(5) As usual, there should be slight tension when pulling the KGD through the coil but not so much that the wraps deform. (No need for "double-insertion.

)
Good luck!
@junny89 and anyone else afraid of the K4 spring: Although the upper part of the base (parts14-24) seems to fit the lower (parts 1-11) in two ways it is like best to try both ways to see whether one screws down more smoothly. Thus, trial and error on this point may have a payoff. IOW screw the upper parts down (part way) notice how smooth the JC plate goes down to its resting point. If not smooth try the other direction.