I installed HID in the stock reflector housing this week-end (3 kits, low, high and fogs). I read A TON about it, and I don't take these things lightly mind you. I took my decision after reading from multiple sources that the Fiesta PASSED lamp inspection with HID in stock reflector. It might not be dot legal, but it is clearly in regulation with how a light beam should behave. The fiesta reflector does not have any light coming directly from the bulb, it is ALL reflected, and while an HID bulb might have different bright points that can cause some differences in the pattern, so does any halogen bulbs to a lesser extent (since they are not as bright). The only consistent light source is an LED, but again, the reflector is not designed for it either, so that point doesn't really stand in my opinion.
After installing, I will have to say that 35W HID in the stock reflector housing do NOT cause any glare, nor does the beam go higher than the cutoff. We did extensive testing while installing them, I really didn't want to be that guy with a ridiculous setup that blinds other people. I can tell you that the beam pattern compared to stock lighting is honestly indiscernible to the eye. When putting my high beams, (also HID), then it does become blinding, as it should.
My opinion on HID in stock reflectors is exactly the same one I have about overclocking your GPU or CPU (I'm an IT guy) : DON'T DO ANYTHING IF YOU DON'T KNOW WHAT YOU ARE DOING. But I will also say that there is a form of elitism going on with people saying "putting HID in stock reflector will cause cancer and end the world". These people are right to a certain point, because A LOT of people that are using HID kits did not do any testing and really are being dangerous with it. However, it is not true that it is automatically a bad thing and that you absolutely need projectors. Just because something wasn't designed with something in mind doesn't mean it couldn't or shouldn't be done. Don't put 55W HID in a 98 Civic with direct bulb vision. It blinds other people and has a glare that's out of this world. However, I will say it with the conviction of a guy that did his homework : 35W HID in stock Fiesta ST reflector housing works, and behave exactly as it should.
Hope this post get some views as it is something I really wanted to say.