A new pack of matched 4200's is faster than a 7.4V LiPo pack. You might see a bit of advantage with LiPo toward the end of the race, since it's got a smoother voltage curve, but the newer NiMH batteries are pretty good. As long as weight limits stay the same, NiMH is probably going to be faster than LiPo in terms of sheer voltage.
But, LiPo isn't about trying to be faster than NiMH. It's about the amazing convenience, the tiny amount of equipment required, and the consistency and longevity. A new 4200 pack will be faster... a 3 month old 4200 pack will not. The consistency of LiPo is king, and one of its biggest draws. Head to head, though, it's not going to be faster, and probably won't get much love from pro racers in competitive events. Luckily, that's about 0% of the R/C population, and nobody but that 0% actually cares about a tiny voltage advantage. It's certainly not worth the mountains of extra equipment required to make it happen.
Now, if minimum weight limits change, LiPo will reign supreme. And, wouldn't you know it, the wear and tear on cars and tires would be reduced as well.