T-Mobile and Google Voice

Ok. A little background. I am currently on Sprint and I have availed of the Google Voice/Sprint integration so that my GV number and Sprint numbers are currently the same. So here's my question. I want to get the Nexus 4 to go with the T-Mobile $30 plan. However, 100 minutes might be too little for me. I typically use around 200-300 minutes a month.

I am planning on using GrooveIP coupled with Google Voice to make the calls but I also do not want to change my number. So how does the integration with GV affect it? Do I still need to port the number to GV? Also, I read that the $30 plan does not support call forwarding from the new T-Mobile number that I would get but would I still be able to make a call through GrooveIP via Google Voice? That would not affect my minutes usage, would it?

Thanks for all the anticipated help!