Well written C++ code will be as fast as the C or FORTRAN equivalent. You will probably find most scientists use C to do serious number crunching (or Fortran, if they have particularly long beards) but that's not because C++ is any slower, it's because they only write relatively small and simple programs and don't need the organisational advantages of objects. That's not to say the algorithms used are simple, but there's no question of writing thousands of lines of reusable code, working in a team, writing a GUI or anything like that - most often their code is written for a very narrow purpose and they are the only ones who will ever need to understand it.
However, I believe there is a lot to be gained in productivity and transparency for yourself if you use objects and adhere to good programming practices. To this end, check out
Blitz++ and
boost.
Bear in mind that you can also call Fortran routines from C/C++; Lapack (which largely supercedes linpack) is written in Fortran.
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