Here's a shot that illustrates the difference between the YS90DX and a true wideangle strobe like the SB102/104 or the Ike DS125 or SS200:

Look at the difference in distribution of light for the two strobes! They both have diffusers on.
Even with 2 of the YS90DX, you will have to be EXTREMELY good at strobe aiming to cover a 20mm lens. For close focus wideangle, you'll probably have to point your YS90DX strobes in a bit to cover your foreground subject.
I hope this is useful for you folks.
James
