Hannes makes a very good point that I neglected to emphasize - for these larger modifiers, they really do excel with more than one speedlite. I usually gang a minimum of two in the 50" Westcott.
It is possible to get decent results with a single speedlite, but it will work quite hard in normal lighting situations and your recycle times may suffer.
Just something else to consider. Remember, these big modifiers are trying convert a hard, 4" light source into a soft, 50" light source. There will be a bit of compromise involved.