If you're recording from an HDPVR type device, yes, you can increase the bitrate in the config.xml.
For any other type of device, the broadcaster is deciding the quality when they encode the content. (and NextPVR is just passing it along - ie, it has no way to increase the quality of the broadcast above what the broadcaster is sending)
If you've got multiple versions of channels, obviously you'd be wanting to use the HD version instead of the SD version. etc.