In regards to playfield plastics warping, I have installed a lot of vinyl siding over the years. With vinyl siding, you have to leave a gap in the J channel in the corners for the siding to expand and contract with the weather. Plastics don't have any room to expand with screws or posts holding it down. Heat is the cause of the expansion but where is the heat coming from? Incandescents could be causing it but it would likely not be just the one underneath a plastic. It would probably be the combined bulbs of the whole playfield. The heat from the bulbs would be trapped under the glass causing a little hot box and making the plastics expand and warp. The proposed experiment with LEDs on one side and incandescents on the other may not be the correct way to recreate these conditions.

Now I can't account for where my machines have been through the years either. So another cause of warped plastics could be that a game was simply left in a garage or shed in extreme summer heat. That would cause plastics to expand as well.

If you click on the pic and press the arrows keys to go back and forth, it definitely looks like the LEDs shine farther. I also like the Cointaker Frosted Cool White for the color. Incandescents make the face too dark and Comet Sunlight is a little too bright and washes out the color. The Cointaker frosted is just right with color and shines as far as the Comet Sunlight. Thanks for doing this, I'll have to remember the results.

Would cigarette smoke yellow plastics as well? I've seen white cabinets turn a terrible yellow from cigarette smoke. It can be cleaned up a little but it seems to stay off white. I would assume smoke would affect plastics the same.

Good Ventilation seems to help this. On my City Slicker backglass there are two 555 right up against the glass where the headlights are but the glass didn't peel there because there is no light board to trap heat. It peeled at the bottom where the displays are up against the glass and prevent air from moving the heat away.

Edit: Now that I think about it, I guess the 555s aren't on all of time either. So maybe a bad example.

I thought your example of Shadow with incandescents didn't look good at all when it came to color fidelity. The colors were a lot darker and Khan's face looked orange. There wasn't a pic of the plastic with normal lighting from the front but I think it's safe to assume that the colors on the plastic weren't that dark to begin with.