It would be hard for me to say that the glass transmitted 100%. I think it probably has a faint tint much as you described, probably 80%. However, it's safe to say that it has no real tint in the sense that people on this forum are asking about. I'm not surprised if you saw some online that are tinted. It's pretty common for dealers to install it as an option, especially in hotter locales.
Here is a pic before the tint. You can see right through both windows and that tree behind isn't much darker.
I haven't really taken any pics after the tint, but it looks great. Maybe I'll snap a few tomorrow.
Slideshow: A one-of-one U.S.-spec Mercedes-Benz SLR McLaren Roadster became even rarer after a factory-backed transformation at McLaren's headquarters.