A journalist at the UK's Car Magazine was told by Wolfgang Hatz last year that the RS5 would get a twin-turbo 4.2 FSI. They ran a very good story about it. A development mule with this engine has been seen on the Ring. From the sound it sounds like a turbo and is distinctly different from noise of RS4 (see YouTube).

That was last year. Since then the world economy has melted down. Suddenly Audi realizes that such an engine is no longer appropriate and so commences work on 4.0-litre TFSI.

Meanwhile, Audi delays choice of engine in RS5 until it decides when to launch it. If Audi launches RS5 this year, then why not simply slot in existing 4.2 FSI engine from R8 with 450 bhp? Perhaps it could be slightly returned to deliver 465 bhp. Development costs = close to zero.

Then when 4.0-litre TFSI is ready, they just slot that in instead. Alternatively, they just delay launch until 4.0 engine is ready and launch RS4 and RS5 at the same time.

Just a theory.