I have an 00 Trail Boss, and stock it would go about 48 mph. I don't think the air filter is your issue here.

That being said, I also have a K&N filter that came with the RCR stage 1 kit I installed. I would not run one without a prefilter over it. Haven't had any issues with dirt or water. Most trails are dusty, but not sandy like the dunes would be. As for water, I know it's not a submarine, and know I have air vents in the lid, so it doesn't get in water deeper than the air box.

I clean my filters after riding, so all that may be needed is a quick refresh spray of oil before a ride.

I plan on buying a uni filter for the wifes Trail Blazer. I guess then I'll have a better idea on comparing the two.

 

i used a k&n filter inside a gutted out air box just used an extension tube made out of s/steel tube. keeps free of dirt and still has good airflo

 

My buddy says the gauze inside his K&Ns is breaking down and falling apart. If he doesn't replace his "lifetime" air filters soon, he's going to start wrecking MORE engines than he already has. I'm sticking with stock, but if I switched to anything different it would have to be the No-Toil foam filter. It works better than the K&N. Better filtering ability, in part because of the super-sticky oil, and even less restriction. I have no use whatsoever for K&N.