Ride it with the plug for sure. You should be able to install the plug and remove it later - or get a $10 non-star plug that just expands that is easier to remove. The manual for my new Specialized with a carbon steerer says:

"Warning! Do not install more than 40mm (1.5") of stack height of headset spacers" and "Do not permanently place stem spacers above the stem [...] defeats the purpose of the expander plug's ability to support the steerer tube and stem."

An inch and a half is quite a lot of wiggle room. I'm basing my measurements off my old bike and will probably ride with a couple spacers above the stem for a week or so to get the fit right.