Next to be evaluated in my search for the best stock motor turbocharger is The Turbo Engineers TTE 550.

The TTE550 uses a center that is similar in dimensions to the RS6 center, but with a slightly larger compressor wheel and clipped turbine wheel.  This center is then packaged in the RS4 K04 housing.

At the current currency exchange rate the TTE 500 shows up new with a price tag of $3,184, not including shipping.  My expectations are high for a product that demands such a price premium over some of the alternatives.