Effect of antenna height on 21MHz performance at GB2GP

The next contest band for HFTA analysis is 21MHz. The GB2GP antennas are either 3 elements on the 100ft tower or 4 elements on either the 80 or 60ft towers.
The graphs below show that the as wavelength increases, height becomes more important to achieve the lower take of angles. the absolute difference between the high 3 element and lower 4 element yagis is less than at 28MHz, but the 4 element antenna at 60ft has better average performance across a wide range of arrival angles, as indicated by the Figure of Merit. 

The general terrain at GB2GP has a downward slope in several directions, with the only rise towards the North East - i.e. towards Japan. This is reflected in the HFTA analysis where the extra tower height seems to really help. You can see the general horizon with the screen shot from heywhatsthat.com below.

On 21Mhz, a lower antenna may be better overall, but my conclusion is that this band will not see any major improvement by moving antennas around.