If you're doing ~20 other FP ops (not including division) per inverse square root and not using a huge dependency chain of inverse square roots, you might not even notice the impact of using an exact inverse square root with this method.

Short of functions like exp and the trig functions, DIY approximations are usually not worth it these days. However, FP division getting faster has made these functions faster, too, since they can now use rational approximations rather than pure polynomials.