A wider wheel span would give a lower centre of gravity and obviously the thicker the tyres the better the traction in the corners BUT i wouldnt want the tyres too wide, they would just create drag on the straights and not give enough gain on the corners to compromise, also think about the cost of wide alloys and the tyres

as for an aero pack for the car, the slots in the rear arches can also be in the front bumper and front arches which would give better frontal downforce, i would but the slots in if possible, a front splitter and a rear spoiler to push the back end down, i wouldnt put the slots in the back arches as it would create more drag than downforce where a spoiler would do the job better.