To weigh in on some of the other questions raised in your original post:
I do not like the look of LEDs in an EM, but that is just a matter of personal taste. Lots of people like them. They are very good for the machine (cooler running, use less energy, last a long time) and they are a 100% reversible mod, so there is no reason (other than cost) not to check them out if you want to see how they look.
As others have already noted, using LEDs for GI in an EM tends to be where you will really notice a much different visual effect, compared to the original lamps. This difference is usually in the overall color and intensity of the light, and in the way the light spreads over the area. Many LEDs tend to create a more focused spot of light, while incandescents generally spread light all over the place. The focused and intense light effect of certain types of LEDs also makes the illumination of the backglass look very different when you use them for GI in the head.
For the PF inserts, LEDs can work really well, as long as the color emitted by the LED and the color of the insert are compatible. A yellow LED under a yellow insert will look good, while a blue LED under a yellow insert will hardly show any light at all. That's because a blue LED produces virtually no yellow-spectrum lightwaves, while the yellow insert only allows yellow-spectrum lightwaves to pass through, and so the net result is mostly nothing.
The same applies for the use of LEDs on feature lamps behind the backglass in the head, such as the ball-in-play and match numbers. This will look good as long as the color emitted by the LED and the color of the screened ink being illuminated are compatible.
There are two unusual visual effects I've seen with LEDs on EMs. The first one has already been discussed in this thread, and that is the tendency of certain insert lamps to flash and flicker while you're playing. This effect occurs when the lamp is supposed to be off. It doesn't happen with every game, but if it does happen to you, the easiest fix is to use a traditional incandescent lamp in those places where there is flickering.
The other visual effect has to do with the use of GI LEDs. Because the GI supply voltage is AC (60 Hz in the USA), many GI LEDs will actually be blinking on and off at a very fast rate. This blink rate (in the USA) will either be 60 Hz or 120 Hz, depending on the internal drive circuitry of certain types of LEDs. In either case, when you are playing a game and your eye is moving rapidly to track the ball, you may see a visual trail effect from these types of GI LEDs due to the after-image of the blinking on your retinas. Whether you think this looks good or not is, again, a matter of personal preference. I find this effect to be very distracting while I'm trying to play the game, so I don't really care for it.