If you do anything to the air box on the 660, you will need to rejet. I tell you from experience. I cut the top of my lid off and went up on the jetting and was still lean. I then put a stock air-box lid on though I had a small hole in the side of my air box, went back to stock jetting and was still lean. The air-box is so resrtricted that the first time you put a hole or anything in it, it let's so much more air in.
My bike has an M4 slip on and its rejetted without a lid. I just ordered a stock lid for the sucker cuz it still runs lean. My old 02 with index drilled mains, glasspack car muffler, and full airbox would outrun this thing...people don't know what they are doing! haha
My friend had the K&N lid on his 660 and had a lot of problems running in Florida because of the water.The K&N lid comes with a K&N water protection thing like an outerwear but just goes on top with velcro so water can still get in your airbox.