> One method of statistically optimising the computation is to remove n least common elements from the set S.

May be you need not remove least common elements from the set, but *get* a set of (len(S)-n) most common elements?

> Does it make any sense to you?

Frankly, not very much.

Note, that least common element is not defined in most cases. Usually there are only a few most (or even one) common elements, but a lot of least common elements. Result of least_common(n) is practically random due to hash randomization.

Another note is that most_common()[:-n] in many cases faster than least_common(n) for n >> 1. This is right for most_common(n) too, but it usually used for very small n (in particular for n=1) and this has more sense.