Highest Efficiency Possible?

After seeing a few threads recently - mostly filled with restrictions for what they're after - I decided to chase efficiency while keeping it reasonable with production cost and time, along with giving it enough power. (Aiming toward the more prestigious end of the car market with the company.)

But I wonder, what is the highest you've ever achieved with an engine that is viable for production and with an engine created as a technical exercise?
Here's that very engine placed into the 2014 Drivers Academy* car:

*The drivers academy car is an affordable car aimed at track day enthusiasts and low budget race teams to allow them to get track day action at a fraction of the cost, and not a fraction of the performance or excitement.
A one make series will support multiple touring car series around Europe.

But I wonder, what is the highest you've ever achieved with an engine that is viable for production and with an engine created as a technical exercise?
Here's that very engine placed into the 2014 Drivers Academy* car:

*The drivers academy car is an affordable car aimed at track day enthusiasts and low budget race teams to allow them to get track day action at a fraction of the cost, and not a fraction of the performance or excitement.
A one make series will support multiple touring car series around Europe.