Assuming you are on Windows.
1) Open C:\Program Files\Serviio\config\application-profiles.xml
2) Scroll down to the profile with id="serviigo_hls" name="ServiiGo (HLS)"
3) Go to the section AlternativeQualities and under MEDIUM rule, change maxVBitrate="800" to maxVBitrate="2000" and aBitrate="128" to aBitrate="196".
4) Save the file (you might need to save it to your desktop then copy it over, else change the ownership to you)
5) Restart the windows service (from the Windows Services management), or just restart the computer
6) Make sure ServiiGo is using the 'Enhanced' profile (Menu->Settings->Profile from within the app)
You may also want to increase the resolution, by increasing the maxHeight value, but remember than transcoding to H.264 on the fly is very CPU intensive, so you will need something powerful to run Serviio at the higher bitrate.
There isn't a way to have more options than 3 at the moment, although you are free to adjust the pre-sets to better match your home and mobile connection.
Audio volume shouldn't be lowered, is it a significant difference?
XMBC isn't compatiable with Serviio's CDS API (which is what ServiiGo uses) however, if you have any other ServiiGo related feature requests, please let me know via the ServiiGo thread
viewtopic.php?f=17&t=6803