I haven't benchmarked this myself but I'm fairly sure that the performance gain you get from using C formatted string functions over the C++ ones isn't very significant.
Stack Overflow - Printf vs Cout
Printf vs Cout - Google Groups
Personally, I would use the stream classes to keep to C++ as closely as possible. They're trying to do fix WPILib to use C++ streams and such instead of the old C functions. They've done it for the most part.
Alex Brinister