I voted suspension because the stock suspension is mediocre at best.. eliminating wheel hop and planting the rear end will pick you up a tenth or two. Converter on top of that = bad ass street car.
A6 cars do not require immediate suspension work like the MT-82 cars do. You can go low 10's or better on the stock suspension and sticky tires with no issues. I would go converter 1st because it will benefit you in any direction you decide to go with the car and it is always on.
here's another option...save the $1500+ you will spend buying and getting a converter dialed in and put it toward a TVS...you wont need a converter anymore.