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
Remember that ambient airflow to an intercooler in the top mounted position is poor compared to a front mount. For efficient heat exchange to happen (and hence lower intercooler outlet temps) you need to have good ambient air flow through the intercooler.
This is the reason I have gone for the 3.5in thick intercooler. 4.5 inches (like most aftermarket TMICs) is a long way for the ambient air to efficiently find its way through the core especially after making a 90 degree turn through the hood scoop.