How are you calculating the slab contribution in the resistance to uplift?
You may also be able to use the continuous turndown for uplift resistance also, for footings at the perimeter of the slab.  
Remember to include a 1.5 safety factor for uplift.

Witchdoc,

You might be drawing exactly the wrong conclusion about your competitors' designs when you say that obviously they know something that you don't.

I suggest that there is nothing obvious about it.  They could just be getting it wrong (eg no safety factor), but you may need to wait for the design wind gust to come along to prove it.

I can recall something similar being said about our designs for circular cofferdams in water when we insisted on providing ring walers with sufficient stiffness (determined by "using classical techniques") to allow for for in-plane instability, when some others didn't.  In the washup, 'others' killed their workers, we didn't.

I strongly advise you to stick with your classical techniques, and make sure that you have sufficient hold down capacity for your footings to comply with your code requirements, (either from pure self-weight or self-weight plus some form of tiedown anchorages).

Both austim and haynewp bring up good points.  Don't assume they are right, perhaps they just haven't been caught!

For "pre-engineered metal buildings" (I just love this term!  It implies all our other buildings are post-engineered!), the soil bearing capacity is usually not a big deal since column loads are usually pretty small.  The critical loading is resistance to uplift (and overturning).  As haynewp noted, a safety factor of 1.5 is required (under most codes).  This factor may be couched in other language such as " the overturning moment shall not be greater than 67 percent of the dead load resisting moment" or other similar language.

Anyway, the way to cover this is to tie as much of the foundation and slab together as you can.  A "turn-down" slab (thickened edge) is often used with a 10-foot perimeter of the slab tied into the edge with reinforcing to create a greater dead load uplift resistance.  Your competitors might be thickening only the area under the columns and not the entire perimeter.  This requires a thicker section and more reinforcement than a perimeter thickened edge approach.  Don't forget joint the slab at the end of the reinforcing or you'll get odd crack patterns.

good luck