I'm using three layers (a 320x240, a 160x120, and a 80x60) now in my blur effect. It does reduce your frame rate but it is based on the fill rate of your video card and doesn't have much to do with the polygon count that is on the screen at the same time. There shouldn't be much of a hit with one layer. You will get a hit with one layer when you use particle effects at the same time as that will require more fill rate.