A great benefit of the clean high ISO ability of modern cameras that rarely gets discussed is how far they allow you to push the output of a Speedlite. It takes a lot of juice to light up a car, but at ISO 800 - 1600 it can be done, even stopped down to f/8 or f/11.
The big catch, however, is that when used in such a fashion, Speedlites give off some nasty, disgusting light. They're a PITA to use for shots like this, and are extremely finicky in terms of getting your angles just right. If not, you get hot spots galore.
Aside from the output advantage, IMHO monolights yield a much more pleasing light quality with far superior fall-off characteristics. They're much more forgiving if your light positioning isn't spot on, too.
As for differences in power, this was shot with two 550EX's at ISO 800 at f/8. I was very difficult to light the rear quarter-panel without the front fender being dark, and vice-versa.
I took a similar shot of the car with some AB800s, and the additional power allowed stopping down to f/11 at ISO 400 with the monolights in the 1/2 to 3/4 power range. More importantly, the light quality is sooooo much better with the monolights.
It's not to say that Speedlites are junk. In fact, I use them far more often than monolights. Even so, just because you can make them work, through sheer will power, in situations they aren't well suited for doesn't mean they're the ideal tool for the job in those situations.
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