That is a neat idea and application! But I fear that like transitions-style glasses, they will either neither get fully clear nor go dark enough. That is one reason I stopped wearing that type of glasses. The main reasons were because they don't work in cars and they take so long to go from dark back to light. Neither of those reasons matter with a motorcycle shield, though.
I wonder why it is not more widely available?
Well, from my experience of owning both, these photochromatic shields actually get slightly darker than the drop down smoked shield that comes with the EXO-1000. Also, it is perfectly clear in low light. I have all 5 of my helmets sitting next to me on a shelf and it looks no different than any of the others with clear shields.
The main thing that sets it apart from drop down shields is that the optics are much better. I wear glasses, so optics are important. With a drop down shield, I have 3 layers of plastic to look through. With the transitions, I have 2. Also, it offers FULL sun protection. With the drop down shield, you still have bright light leaking around the sides and near the bridge of the nose. Finally, it transitions automatically. There's no having to mess with a switch when you go in and out of shade coverage.
I was also hesitant about purchasing it because of the price. However, having owned it for a full ridding season, I would replace it in a heartbeat if it ever got damaged. No longer do I have to carry spare shields or have to fiddle with a drop down visor (which makes a helmet thicker than it needs to be). I wear the transition shield in the daytime and nighttime.
You are correct about sizing. Scorpion helmets run 1 - 1.5 sizes smaller than most helmets. My EXO-1000 is a size 3x and it's slightly tighter than my 2x RS-1. Also, I can fit easily into a 1x Shoei Quest/RS-1100. Bang for the buck, Scorpion helmets are hard to beat. I own 2 of them

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