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.
Nov 15 2019:
Looking at the models of OCFDs again - I realized due to the lobe formation being either side of the X or Y axis on 40m and above, the far field plot does not show the peak gain as its simply sliced down the X axis. The geometry would need to be offset such that one of the main lobes aligns with the X axis. Comparing the elevation and gain results on the calculation tab with a plain 40m dipole vs an 80m OCFD on 40m band, the OCFD has around 1 dBi more gain at the same elevation angle (52 degrees when 12m above ground), which comes from the two broadside nulls that form. Under far field plots, the far field elevation angle can be changed, comparing gain at 40, 30, 20 and 10 degrees shows the OCFD having about 1 dBi more than a dipole at the same height.
I'll need to update my table..