Thanks, everyone...
After rethinking this, and considering the comments, I've decided to go with the stock sizes for now. Next year, who knows? At least I'll have a baseline for stock size tire handling. Handling and stability at highway speeds was my primary reason for the larger tires along with the apparent effect of a 15.5 tooth sprocket.
jonathan180iq - You called it spot on with the Shinko rep's reply. However, he also said he knows of many GZ's using the larger tire size I was considering without any issues.
JohnC - That wheel/tire chart is great! Thank you!