What Baron has already said Russ, it's very important, and honestly, without it, it's an oranment.
A small 20mm gurney will massively increase the downforce, and really, creates bugger all drag, because all is it doing is creating a high pressure zone on the top 20 to 40mm of the wing's surface, pushing it down.
Without it, all you have is a blade running through the air, and when you increase the angle of attack, you are creating as much horizontal drag as you are downforce. Very inefficient.