I am not having any trouble with call quality. I wonder if you have something on your router that's blocking your obi device? I don't think it's related to your use of sipdroid.
Also, I tried obion for android, and it seems to work fine, but it's kind of cumbersome to call your own obi gizmo and answer all of the prompts to make a call to a phone number. Sipdroid works directly with google voice, so it's much easier to make a call. I don't need to receive calls on Sipdroid, so it's not a problem, but I'd be interested in having a post of the solution for dual sipdroid/obi ringing when you find it.
My biggest worry is that Google will mess up my system because of the request I submitted to transfer my number to a different account. Google voice is a great service, but if anything goes wrong there is really no way to get support. Support from them is a complete black hole.