This is just a one-off for now, but it is a very interesting one nevertheless. And they are apparently contemplating building more if there is demand.
http://www.autoblog.com/2012/06/17/p...wer-six-speed/
They don't mention whether the displacement is increased from the stock T6. It doesn't sound like it. Assuming this is the case, for what it is worth, it makes 508hp / 2.953L = 172hp/L. Even if they drop it down by 10% (say) for production, they are still over 150hp/L.
On a side note, that blue looks fantastic.
*Note: There is no F8x M3/M4 vs. forum yet, so this forum will house such threads for now.