To build a picture of what one might expect when considering low band DX antennas, I feel it's better to consider the gain at different wave angles.
To illustrate this, I created a table in a spreadsheet with several different antennas and the gain they have at different angles. LiberOffice Calc has the option to fill cell background color based on the value. The table is sorted based on the gain at 5 degrees.
I used MMANA-GAL for the modeling of these antennas. Most are 40m antennas, those that can be matched on 80m are also shown.
Wire antenna heights are 40ft/12m, the inverted Vs are 15m/50ft, the delta loops have a minimum height of 1m/3ft.
From this table I can see that one of the simplest antennas gives the best bang for buck for DX on 40, thats the half square. A small surprise is the ZS6BKW on 40m looks to have an edge over a 43ft vertical for DX! The OCFD antennas are well behind.
Disclaimer: I'm no expert at modeling antennas, I perhaps know enough to be dangerous :-)