If anyone is mildy interested I did some tests today and found both versions of the removenth function that use vl-remove-if were approx. 2x faster than the vanilla version, using lists with 1,000 to 1,000,000 items (using a representative spread of 'n' positions).
Having said that, that isn't terrible performance for vanilla lisp.
Challenge extended -- writer a faster variant in either vanilla or visual lisp. Hint - recursive will look prettier but ...