This is a highly tuned engine, if you try to apply forced induction you have to buy new pistons to drop the compression, new custom exhaust manifolds, different exhaust, intake modifications, plumbing, turbos, injectors, fuel pump, pressure regulator, plus ecu programming, not to mention drivetrain changes. On an Italian car that this kind of modification is not often done to, its gonna be expensive. 20k is a good deal. By the time you're done its probably easily closer to 30k.
Take a look at the highly tuned W12 NA that audi uses, vs the W12 twin turbo in the Bentley continental, and look at the performance number differences.
The W12 is essentially two VR6's which is a very efficiently tuned engine to begin with. The Bently engine only makes like 50-60hp more than the NA version, and I think like 80lb-ft of torque more, however across a much broader torque curve. This is because just to start pushing the boost through the motor, you have to drop the compression from 11:1 to closer to 8:1 generally. Just doing that loses a good amount of power, but add the boost and you make that up, and with smaller turbos get a nice fat torque curve. Sure you could run bigger turbos, with a really open exhaust system for some really high horsepower numbers, but your torque would drop off, and it would be a rediculous car to drive.