Need more power

Having owned the S8 for about 2 years now (and my 400bhp scooby is almost finished), I getting the need for more power. I'm thinking 50bhp plus.

Apart from the K&N filter the engine is standard.

What are the options ?

• I assume an exhaust will free up a few horses but nothing special
• I'm really loathed to fit NOS to the car, even though I have run it on most cars I have owned over the last 15 years.
• I have found a lovely supercharger from various places, the MotoDyne looks excellent at $40k
• An RS4 lump with turbos
• Or is the top end swappable

Interested in your thoughts but more in things that have been done already.