Once more the S54 engine tuning saga
The P54 engine by BMW Motorsport (~80k euro just for the engine) produces 390-400hp and is the racing version of the S54.
The reliability of course is the one of a racing engine.
I don't understand how someone can match these numbers without drastic changes and relevant cost of course (Nathan said 20k euro for non-BMW upgrade I suppose ?).
My personal opinion is that the last thing a road/track CSL needs is engine work.