There are some good ways to stiffen up the suspension and lower the ride height on your Audi...cutting your stock springs is definitely not one of them. You can buy lowering springs for roughly $250 retail that will do the job nicely. If you want to get more aggressive, for $1500 or so (there is a lot of variation in price depending on exactly what you want) you can buy coilovers which give you many more options for fine-tuning your suspension. Both springs and coilovers, when properly installed, will provide the performance you're looking for without sacrificing safety to achieve it.
1. Your shocks will now be too long, and will bottom out
2. Some springs are progressively wound. Not sure about yours. You may not get the spring rates you expect
3. At best, you are at trial and error
4. If you cut springs, the % by whcih you drop the height and the % by which you increase the spring rate are identical. You may -- in fact probably dont - want that. You might want to increase rate by 30%, while lowering the car by 15%. No can do that way.