Not sure about that. Front and rear bumpers just replace originals, so no extra there. Side skirts are extra but weigh very little, and half the original metal arches are cut away to fit the fibreglass arches.
265 wide tyres going to give a lot more grip than stock wheels, giving better acceleration. Profile should be the same so wont affect anything that way...the only thing is that the wheels probably weigh more. But I would expect the grip to far more than make up for that.
Plus swb quite a bit lighter than lwb (more than bodykit and wheels weigh)
5.7 secs is respectable, although zed with chip, exhaust, filter and 13-14psi boost can get under 5secs