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.
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.