I can only relate my experience in compressing to 25g with and without the HD audio. The higher the compression rate, the lower the quality of my copy. Thus, if I can achieve a compression rate of only 20% or less with HD audio enabled, I go for it. If the compression rate with HD audio is around 40% or higher, I disable the HD audio, which lowers the compression rate.
In my experience the higher the rate of compression, the lower the quality of the final copy. Thus I will trade audio for a lower compression rate.
I hope this helps. Also, if my advise is incorrect, hopefully someone will chime in.
--rjl2005