Is it possible that what are seeing on a sloop is actually the mainsail being "back-winded" by the jib (air flow) rather than luffing. Since the 16 has a fully battented main, you rarely see the main luff or get back winded.

Other than that I have never noted a difference between a monohull or cat in which sail luffs first. I would always be looking for the jib to luff while going upwind.