I have asymmetric hearing - does this compensate my high frequency loss or mainly boost overall volume?

After the created hearing profile is applied, it will compensate on your lost frequencies. If the testing has correctly identified your hearing characteristics then it should compensate according to the same, you should be able to hear more of the frequency range which you had difficulty hearing previously. The audio compensation algorithm employed by the headphones allow for asymmetric hearing and will compensate for each ear individually. For some users who have become 'accustomed' to hearing asymmetrically - this may become a new 'norm', and sounds like overall volume boost. In these cases when the audio is 'corrected' by the compensation algorithm it may take some time to get used to it.