according to
http://msdn2.microsoft.com/en-us/library/ms880945.aspx there are 2 registry settings that can control the volume of bluetooth headsets.
adding the \HKLM\Software\Microsoft\Bluetooth\AudioGateway SpkVolume=0xf(15) value may improve it (15 seems to be the max, it gets reset to that if set any higher). However, that's not how I ended up fixing my own motorola HS850; somehow by re-pairing my headset to another device (my macbook), it fixed the way too low speaker volume.
My best guess is that the max volume gets set during pairing by way of
http://msdn2.microsoft.com/en-us/library/ms881893.aspx The volume of the headset's own beeps seemed to be affected also, including its max volume beep. I haven't been able to re-mess up my bluetooth volume, though, so I can't say for sure if any of this explains it, but it might be worth a try. I haven't had a chance to experiment with the mic volume setting yet.