Those Speedlite are some very powerful units.

The measurement they use to get a GN is pretty standard - the only thing you can do to a speedlight is lower it's GN with modifiers.

A studio light however, has all different sorts of modifiers and that's kind of the intent of a studio unit - so it's not really possible to compare one to the other. The closest you might get is if you could use similar 7" parabolic reflectors, but I bet even that wouldn't give reliable readings.

True Watt Seconds is the truest measure I've been able to compare. "Effective" watt seconds is way too subjective. And even all the charts and graphs can lie, er, be manipulated and interpreted to show any light as being the best at something.

300ws (true ws) is sufficient for most indoor portrait and wedding uses, even for groups (figure you'll have multiple lights for main, fill, hair, etc). 160 ws (AB400) is a bit weak for some uses IMO. 600ws can be useful, but for many things you'll have it turned way down, and the cheaper lights won't be consistent in output and color from shot to shot.

For outdoor work 1000 to 1600ws is where you need to start if you want to overpower the sun on anything other than one person.

Personally, I can't imagine why you would want to "overpower the sun" with location lighting. Steve Dantzig takes a Dynalite and battery pack and a softbox out on locations and gets awesome results with a 600 Watt Second unit.

I Use little Nikon Speedlights as I said B4. They are powerful, but I don't know what the WS rating is on them, and I would guess around maybe 200 or 250WS. The thing is, we aren't trying to overpower the sun, but using the sun as a backlight, and balancing the light on the subjects. I recently photographed a family reunion with 40 people out in a park. Used one SB800 and 2 SB600s. Came out great!

I regularly photograph families of 7 to 10 people out on the beach here in Hawaii using one SB800 and one SB600. Beautiful results, perfectly balanced to the background, or I can even overpower the sun and make the background dark...but I prefer not to, as the background is beautiful!.
ALOHA!

From what I've read speedlights are 50 to 100 ws range. You'll find this out if you start bouncing them into umbrellas compared to 'real' strobes. When I played with mine I found I could get maybe 2 stops of light out of the bounced deal, but 300WS monolights could give me 6 ot 8 stops. Handy if you need to overpower ambient by 3 stops to elimiate the color cast of the room lights!

I've seen simulated sunset shots done by overpowering the sun, and unless you can get the light close to the subject you need a LOT of power.

When I got my vagabond I played a bit outside in the noon day sun - overpowering a single person head to 1/2 shot can be done with a direct speedlight, but try 3 people full length - a very different situation requiring a LOT more power. And if you want soft diffused light then you'll need a lot more power up front.