epalermo:

You'd have to see if Wonderboard is a code approved sheathing and if the manufacturer recommends it for that use.

What scientific basis do you have that a Wonderboard sheathed wall is more fire resistant than one sheathed in OSB or plywood?

WonderBoard should not be used for exterior wall surfaces with a wind loading greater than 30 lb/s.f. and not as a structural load bearing member. It is usually used over structural panel sheathing as a substrate for cement stucco.

WonderBoard is non-combustible so it will be more fire resistant than combustible sheathings like plywood and OSB but fire resistant wall assemblies also use gypsum board to achieve a tested fire resistance rating. When exposed to fire, any kind of gypsum board releases water vapor that cools the surface. When the gypsum has released all of its water vapor it must then meet a hose stream test which is why it needs fiber-reinforcing found in type X and C board. UL tested fire resistant wall assemblies with WonderBoard that I have seen also use gypsum board on each side of steel studs.

All materials other than poured-in place concrete will eventually fail structurally or be consumed in a severe fire. The task of a fire-resistant assembly is to delay that process until people can escape and/or the fire service can arrive.

I don't believe an exterior wall faced with wood fiber & cement siding has passed a UL test but it will be more fire resistant than wood siding. An even more fire resistant cladding would be cement stucco.

Will the studs be wood or steel? Is a fire resistance assembly rating required by a local ordinance or your insurance company?

Fires usually occur inside the home from smoking, unattended cooking and electrical failures. Severe fire exposure from outside is rare except in a forest fire. In that case you should also be focusing on the roof and the windows.

The Laguna Beach Firestorm of 1993 demonstrated that some combinations of materials work better than others even in catastrophic conditions.

Here is a link that might be useful: Fire Resistant Details