Hi Brian,
I have a K&N RU-1770 mounted on my Pred and with the right jetting, it is running extremely well...
I am using the Dynojet stage III kit set-up, with a DJ 175 main jet, the DJ needle on the 2nd clip down from the top, a Mikuni #50 pilot jet, and the mixture screw out at about 3.5 turns. I had the W/B slip-on pipe, and I run all 8 discs with the quiet core insert at the end. I also use a 14/38 gear combo. I ride at sea level up to 1500' +/-, in warm sunny SO CAL, usually at Glamis Dunes. The motor pulls extremely hard from bottom to top, which on my Pred is 8500 rpm. There, the stock piece of crap CDI box start to retard, and induce high end popping.
I have a photo of the filter on the quad in my member page...jjgi5150.
I will be purchasing a new Vortex CDI box in a couple of weeks, and I will report on the difference once that is installed. I mounted a quality Autometer tach, and have now learned how to shift the motor right at 8200, to miss the CDI rev limiter. Driving it this way, it just plain hauls butt to no end. I am extremely pleased with the jetting and performance now, but I am also looking to change to a Yoshimura exhaust system soon.
Good Luck,
JJ