You would cut 1/2 the amount you want to lower the car. Handling would improve slightly from the lowered CG.

But that's really not the way to do it, as others have pointed out. With less travel available, you'd want stiffer springs as well -- and stiffer springs will improve handling, even at the stock ride height, more than lowering the CG.

Vintage Performance Developments has the springs you need. You will also need to complement the stiffer springs with shocks that 1) match the stiffer rate, and 2) operate properly in a semi-compressed state.