What do you mean at a guess? Plenty of people running .82 housings on GT30's and loving it. Do you think the smaller housing will feel the same on boost? There are tradeoffs, namely torque and top end response. That's why you need to match the turbine AR to the engine size, and where you need the power in the rev range. Every build is different.
I used to think smaller housings were great, and they are in some cases, but after running the 1.06 I realised there is much more to large turbo's than trying to spin them up quickly with a small nozzle. Let the engine breathe, don't strangle it, or you may as well just shove a smaller turbo in there for better response.
Now is the time to swap if you are going to, seeing you need a re-tune anyway.