It's not going to make a world of difference, but it will make a difference.
Everybody always talks about turn 8 with aero, but it's the one corner at Willow where -- if anything -- you're going to be slightly slower with aero (although in reality, you'll probably be just about the same). The reason for this is that with our cars (assuming your motor isn't sneaking out huge power), you're flat through 8 no matter what. (If you're not, then you've got your car set up maybe looser than it ought to be?) But if you're flat without the duck, then the only thing the spoiler is getting you is drag.
But aero will deliver dividends at Willow in turns 1, 2 and 9. Those are corners where most of us are not flat on the gas, and the 80+-mph-generated lift is actually making a difference in how the tires hold. Spoil that lift, and you can carry more speed through those corners.
To me, 2 is the corner you want to do all your setup for at Willow. When I run my back-yard hooligan wing, that's the corner where I do my adjusting.
All that said, your VARA cars are light as a feather, twitchy as a teenager -- and you guys build 2.5 liters to pretty healthy numbers. So flat through 8 might not be a given. If that's the case, well...
...then I'm freakin' jealous. 