If you were considering whether to use /fp:source vs. /fp:fast, you would probably add additional flags to narrow the differences, such as -Qftz -assume:protect_parens -Qprec-div -Qprec-sqrt. You should look up those individual options; /fp:source by itself turns off ftz and should set those other options.
If you allow your debug option to set /Od, it will eliminate most of the aggressive effects of /fp:fast. If you set the same /O level as your release mode, debug will not change the effect of these options.
The most visible effect of /fp:fast=2 is to set complex limited range, where the valid exponent range for complex is cut by half. This would permit vectorization of many complex operations, but isn't worth considering if you don't have vectorizable complex.
The most visible effect of /fp:fast -assume:protect_parens -Qprec-div -Qprec-sqrt is to permit vectorization of sum reductions which aren't permitted under /fp:source. Vectorization of reductions usually gives a slightly improved accuracy which may vary with data alignment.
