Adding rear aero, assuming it's even remotely functional, is going to add understeer to the front on the faster corners. That's going to make the car feel very sketchy, especially at the Ring when you think it's a speed circuit (as opposed to somewhere with lots of slow corners). Unless you know exactly what you're doing as well as knowing the Ring very well (50 laps or so), or have had the chance to have a play on a track beforehand, I think you're simply going to make the car worse for you.
Much better to have a car you know very well at a place as dangerous as the Ring, instead of a car you're experimenting with and have no faith in. You'll be really irritated if you end up crawling round the track because the car doesn't feel right, rather than just being able to enjoy it and concentrate. A car that feels weird is going to increase your likelihood of binning it, rather than reduce it.
By all means book a trackday up beforehand and go play with a BGW, but please don't do it there for the first time.