I did a few long rides (6000 miles) with identical 2010 990R's same exhaust, stock R map with the unifilter prefilter on one and the other with stock snorkle. They got nearly identical gas mileage, no noticeable difference in performance. The bike with the prefilter required it to be cleaned several times during those rides but kept the main filter cleaner longer. After the 6000 miles the more frequent maintenance became a PITA. More work overall for not much benefit. After this first hand experience I decided not to install one and he was going to remove it at the next filter replacement. YMMV.
Born 10,000 years too late
KTM 2010 990R, 2013 500EXC