SAAB'S fastest road car ever is the new 9-3 Viggen. But the company goes to pains to point out that, despite temptations to do so, it should not be compared with a BMW M3. The Viggen, Saab maintains, is a high-speed cruiser in the Gran Turismo tradition, not a race car. It fits that role well with a major reservation - its power and torque are such that the front-drive chassis is stretching itself to contain the resulting torque- steer.