I found that I preferred the very slight sound differences I experienced when my gameboy light was still working.
What I noticed as significant is that the wav channel had a more midranged sound playing the same synth. One problem that can occur with a lot of DMG songs is that they get very bass heavy and this helped that.
I used to write and record a lot of songs, for 8bc etc, only on the gameboy light and with the light on and noone seemed to notice.
Also, I've only seen photos of EL backlit DMG's but the light on one of these is just amazing.
About your original question, I remember that Nonfinite posted a topic on 8bc a while back where he'd opened up a gameboy light so he might have a bit more insight into prosounding one of these.