It just looks like standard fill flash to me.
I suspect your technique could use some improvement. You say you don't take advantage of your available sync speed.
Moving from 1/100 to 1/250 will give you the equivalent of a 2.5 x increase in flash power.
Another important consideration for maximal use of the power you have is that the flash is not illumination an area far wider than your camera is seeing. This is very important for outdoor flash photography where distances are generally larger and you have less objects (walls, ceilings, etc) for the flash to bounce off. A fresnel lens j front of your flash can do wonders.
If your shooting into the sun, some flashes can be fooled into producing less light. Are you manually setting the flash power?
However if adding capacitors is what you want to do (as opposed to solving the problem with what you have) then I recommend you get flash rated capacitors.
Flash rated capacitors are designed to discharge quickly and are this able to give a shorter, brighter flash. Just adding normal capacitors may result in more light, but the flash duration will be substantially longer (but not even over that duration). If you want the flash to stop motion you may find you're still getting motion blur.