A few weeks ago i ordered a 7,5meter TTL cable because i wanted to try some things out with very shallow DOF combined with flash light, the problem with the shallow DOF is that with the use of very luminous lenses and large openings (f1.4 / f 2 ..) there's a lot of light coming in and this raises the shutter speed way above the max sync speed of 1/250.
All pictures (except one) where made with the 85mm f:1.8, nikon SB-900 behind shoot trough umbrella on a light-stand.
The flash can be operated in full manual mode while retaining the high speed synchronization possibility (or TTL if you prefer)
Above 1/1250 -1/2500 there is a significant loss in light hiting the sensor.
The strange thing is that when the SB-900 is in normal mode and you set the power level (M) to 1/1 aka full-power it takes about 5 seconds to recycle after firing which is pretty normal, but when you use the SB-900 in the high sync speed mode (auto-FP) @ full power, it takes no time to recycle so it seems to be using a lot less power...
(You could also make use of ND filters on your lens to keep the shutter speed low while using large opening)
(This one is ambient light only)