I have already seen photo-cells aimed at the windshield, but 5-8cm of diference is nothing, because both start and finish sensors will be at the same height, so they will be interrupted by the same part of the car, so I dont think its a big problem, and the best cars have 600hp tops.
Maybe the laser pointer plus sensor is the best thing, should I use LDR(I heard that those are pretty slow, or IR(maybe red lasers trip then hopefully 
), or another?