straight cut gears are stronger pal y would they be used in motorsports if they were not stronger and better at transfering power/torque
it depends on the overall design of the gear and the material of which it is made, but on the whole helical gears are much stronger and transfer torque much better then straight cut or 'spur gear's..
helical gears are stringer because there is more teeth meshing at the same time in the gear set, which means much smoother torque transfer and less shock loading and stress on the gears as they each share part of the load.
spur gears on the other hand only have two teeth in full contact at one time, so those two teeth have to transfer the entire load between them, putting more stress and load on them. because only two teeth are in contact at one time the torque transfer is allot harsher as there is a gap in the rotation of the gear set in which there is not full contact between two gears.
the reason they are used for track / racing cars is that they make gearbox servicing and part replacement 10x easier. as im sure you are aware helical gears mesh into one another with opposing cut angles where as spur gears use symmetrical cut thus the race teams can design the gearboxs so that each shaft can just slide out of the casing saving time of pulling the whole gearbox apart for servicing or replacement parts.
down side to helical gears is as above it takes longer for gearbox servicing and parts replacement as you have top pull the entire gearbox down, and they are much more expensive to produce than spur gears.