The CreateCollectionOptions does not currently support the "noPadding" flag, which is important, since "usePowerOf2Sizes" flag is deprecated/ignored since MongoDB 3.0.
I believe it should be implemented, and probably should produce some kind of warning or even an exception if the underlying database instance is version 3.0+ (throwing exception is of course may be going too far, so this is only a suggestion).
Without this flag, it appears that there is no API-supported way to disable padding in mmap-based storage engines when using newer versions of MongoDB. Issuing the "collMod"/"createCollection" commands manually of course would work, but it looks more like a hack.