Discs are too big to fit 15s. They might make 15s with a 114.3x6 pattern but why on earth would you do it? Unless you use a stupidly large profile tyre, your axle height drops, reducing your ground clearance.
If you fit smaller front discs you'll probably have braking problems and if an engineer saw it he ought to reduce the towing capacity too (since towing capacity is a function of chassis strength and braking capacity).
The D40 in Australia comes with 16" rims and 70 profile tyres so that the wheels can absorb some of the ruggedness of our roads. The European Navara comes with 17" rims and lower profile tyres because their roads are a lot smoother - I don't know if it's a continent full of pussies or just smarter road building (or more money invested in roads) but they can get away with the lower profile tyres on larger rims.
The larger rims would make the vehicle corner better - less tyre wall flex - but since we aren't on a race track, that doesn't matter as much.
