Not too surprising that someone implementing a web browser would research other implementations, especially open source ones. He also could be wanting to see how to integrate V8 into RockMelt, or some other component they have no need to rewrite from scratch. Don't know if that would fit your definition of "based on".
C# is managed code. At the least you have to pay for automatic garbage collection, array bounds checking, etc.. So the most optimized C# will be slower than the most optimized C. Just make sure your C code doesn't have any memory leaks.:)