The OEM mesh windscreen can stay on with the top up.
I checked out all the windscreen offerings for the XLR and purchased one from http://www.windrestrictor.com/ and am happy with it. The OEM windscreen was just too expensive, and there was more negative feedback than positive when compared to the other brands offered. I didn't get their fanciest model with custom laser engavings and brake light activated LEDs, since that wasn't my style, but got the basic clear version instead. There is minimal glare and nothing of note on the rearview mirror at night, which I was concerned with. The design cuts the turbulence down around 50%. You can find a lot more detailed offerings at the other good XLR site : http://www.xlrforum.com. Hope this helps.
Automobile(s): '08 STS-V Sedan Black Formerly three CTS-V's, XLR and STS4N*
Re: Goodwrench wind screens
We have the factory windblock and like it. It is easy to remove and it can stay on full time. IMO it is much classier than a piece of plexi stuck on the roll hoops. Call around many dealer parts departments will order it for you at a substantial discount.