Efficiency is one thing. Final cooling charge is another thing. While a smaller unit is probably more efficient by nature, it probably will not have a cooler output temperature.
There are results to back this up as well. If you look at the APS units, they sell the 525 and 725 units. The 525 is for smaller turbos and the 725 is for larger. While being more efficient at cooling, the 525 showed about a 5 degree warmer charge than the 725 at the same horsepower level.^1
This is also why they say, there is no replacement for displacement. You can be efficient all you want but bigger is usually better.