A PSU will be most efficient if the load is close to what it’s designed for. A PicoPSU doesn’t have as many components as say a full ATX PSU and they are all sized for tiny loads.
I have used Pico PSUs on several occasions, more so due to their size than their effeciency. So I don’t think I’d swap a normal PSU out for one. With the difference of just 10 watts you’d probably have to run it 24/7 for years to break even in the cost of purchase.