Hi Garry, I get what your saying;
Overpowering mid day sun isn't that hard. To do it well (easily?) you need to start with a suitable camera and the right means of synching it to the flash.
First of all, you need a camera that will work at a relatively high shutter speed, because obviously the shorter the shutter speed, the less effect the ambient light will have at any given aperture, and the less flash power you'll need.
The ideal choice has to be a medium format camera with a leaf shutter, or if you're using a small digi camera then the old Nikon D700, which can be used at high shutter speeds, is worth thinking about - if it will produce the image quality you need.
Otherwise think in terms of a camera that will synch reliably at at least 1/250th.
Completely understand what your saying here, however I have at my disposal a 5DmkII with a max sync of 200th that will have to do, I'm done messing with film, and I'm unfortunately in no position to be buying medium format digi backs

I know its not the best, but something I'll have to work with.
And use a radio trigger that doesn't have a delay in the circuit, i.e. a PW. If you haven't got one, use a synch lead instead.
Not a problem, got some PW's around, and I'm not scared of cracking out the old synch chord
Next, what type of modifiers will you need to use, and at what distance?
Softboxes don't actually eat light but they do spread it around enormously, so if you can avoid using them then you can manage with less raw power.
If you're mixing ambient with flash then you can usually get away with beauty dishes and/or high intensity reflectors, both of which are very light-efficient, especially when used at short distances.
Possibly on occasion a medium size octa, in reality on in the early morning/evenings as like you say you tend to loose control of the light in very bright conditions with softboxes. I'm likely to stick with the high intensity reflectors and beauty dishes 90% of the time.
Given a fairly short shutter speed and the means of getting the most of it, you'll probably be able to manage with 600 ws. Less than that may be pushing your luck.
I have to agree, I don't really like the idea of dipping much under 600w, which is why the quadra at 400w puts me off, or I would have a set tomorrow...

Now what the difference between 600w and 400w in practical terms means, I dont really know.
Next question - how short does the flash duration really have to be? I don't think that there's a system on the market that can really freeze closeup dancers, but often it's good enough just to get most of the subject sharp and acceptable or even better to have blurred hands and hair. If you really do need the shortest possible flash duration then you may have to look at the Profoto b, but they aren't cheap, although you could consider hiring instead of buying unless you need to use them frequently.
This is a good point, and one I hadn't really thought about... I probably dont need a super fast duration, more like wanting it for the sake of it as often happens. Profoto B, most likely out of my price range, and to be honest I doubt I'd make full use of them. The Hensel option does look very nice, but god knows where in the UK to get hold of them lol.
Thanks for taking the time to reply though Garry, really appreciate it.