Stephane --Helical gears are superior to spur gears. Beside the quietness, the load carrying capacity is far superior with an helical assembly as the mating surface is obviously wider with an helical and also because the load distribution is progressive.
I'm not sure your argument holds together, but perhaps we just have some terminology issues. First of all is "width" of mating surface really the right parameter? Don't you mean "area"? Also, don't you need to consider the effect of frictional losses and heat generation from the "progressive" load distribution? Load carrying capacity, after all, needs to be seen in at least two different lights: instantaneous and long-term. Are you arguing that a helical gear set has both higher resistance to shock loads such as clutchless shifting as well as being able to tolerate higher power transmission over the long term? Always, under all conditions?
If you have references to some reputable technical articles that support your assertion we'd be all ears. The generalization is just a little suspicious given the predominance of straight-cut gears in transmissions designed for motorsports competition. Can you explain?