Coss,
How much headroom do you see people losing when they add a sunroof? I know the sunroof, if the glass can be removed or if it goes into the roof like in my truck, takes some additional space. I don't imagine the Elio roof will be any thicker from outside panel through the headliner.
If it's a "pop-top" sunroof (the kind that vent up or you can take the glass out) the inner frames are only 5/8" to 3/4" thick (deep). A power moonroof (the kind that drop down and slide inside are 1 3/4" to 2" thick (deep) and you have a pan that is 2" to 2 1/2" wider than the glass on the sides, 3" to 4" wide in front of the glass (that's the drive assembly / drive motor) behind the glass it is the front to rear size of the glass, plus 3" for the glass "tray" So say you have a moonroof glass that is 1' x 2' side to side (just to make the numbers easier) the pan that mounts to the underside of your roof is 28" front to rear and 28 1/2" wide side to side.
There is another glass roof option; the kind that pop up in the rear, and slide back on the outside. The factory option can give you a Huge piece of glass on the outside, but the opening space on the inside is only a 1/3 the size (at most, more like 1/4 the size) front to rear and on the sides you lose 4" to 5" on each side. There is another option for a sliding vinyl sunroof like a lot of UK and EU cars had / have; it's a great size hole, but it's only a sunroof when it's open, the rest of the time, it's a solid panel over head. Not a very popular option.
Excuse me while I brain dump here, you got me started and I just couldn't stop myself.
Your lucky my cat just jumped out of my lap and left some nice puncture wounds in my leg; it was enough to distract me