Bach M in B minor Kyrie and Mozart M in C minor Kyrie - SATB solo, chorus, and boys?

For liturgical purposes both Bach an Mozart would have been accustomed to all-male choirs, similarly constituted to the Gentlemen and Boys of the Chapel Royal in this country (UK) even today. (Bach in fact was not infrequently upbraided by his employers for giving, in their view, insufficient training to his choirboys.) Rather than using the term 'authentic', a performance of either of these works using boy trebles would probably result in a sound picture that more closely resembles one that either composer would readily recognise, given the opportunity.