G'day Rolf,

"I have trouble finding a canonical explanation of Orcish Manoeuvre, ..."

"A Fresh Look at Efficient Perl Sorting" describes many sorting techniques. The section on OM points to a footnote with details of a book by the inventor of this algorithm. I don't own, nor have I read, this book, so I can't comment further.

"... but isn't Memoize faster in this case?"

Try Benchmark to answer that. Please post your results: I, and no doubt others, would be interested.