Wrong big time ,,if you use the HF coil as your primary coil ,your finds rate will drop,, HF coils are sensitive to very small surface and shallow targets ,Im in the UK i compared a nine inch HF coil with the nine inch stock coil , ie, 40 hours out in the field ,, with my detecting colleague ,,,and my findings were,, there was nothing that I found using the nine inch HF that a nine inch standard coil would not find .
13 inch X 11 coil is heavy and you will need an after market block to centralise the coil to the shaft, which makes it even heavier ,if you measure the curcumferance of the 13 x 11 using the ellipse formula theres not much differece to the 11 inch round coil..