How is the balance now? Also, how low is the front of the car?
You might be going too far. Your current wheel rates are ~250f/r and you are looking at 675 front and 495 rear. That is not only a huge change, but also quite a balance change.
You might try to contact Dave Feighner or Marcus Merideth, they both have/had bad ass SN95 cars. I know Marcus/Jenn's car was using a 1600 or 1800 conventional front spring. I don't remember the rear rate. That car was plenty stiff and drove fine (It has a stack of National trophies, including one I won in it) Using MM's formula that is a 400-450 wheel rate in front.
If the car is lowered too much, which almost all of them I see modified are, there isn't a spring stiff enough to fix the front. IIRC, the roll center goes 18 inches under the ground and is therefore 3 feet away from the CG and it just sucks.
So, without knowing all the details on your car, I would supply this generic advice
-Front springs in the 400-450 range.
-Add the Tall Steeda balljoints.
-Get the ride height up.
-Raise the rear rates the same percentage you raise the front to start, adding or subtracting 50 lbs to adjust balance if desired.
Hope this helps. I am making a lot of assumptions that may be very far off since I haven't even seen the car. My overarching thought is that your proposed rates are too stiff for a radial tired car. I have run rates in that range before on my old CP car, but that was on the old floppy bias tires circa 2006. I don't think that will work on an A6, or even the newer design bias slicks.