Big Integers

With the csc compiler and the -f or -fixnum-arithmetic option (Assume all numbers are fixnums) my benchmarks appear to be quite fast compared to racket of chez scheme.  When running a benchmark that uses big integers (such as the pollard rho), execution times are almost twice as long as racket or chez.
Is there an egg or alternate code to improve the efficiency of big integers?  I would have thought there would be interface to gmp.