Noise damage (which isn't going to equal for everyone as there are things like genetics etc which can also influence things, so what is "safe" for one person might result in damage to another) is basically a product of time and intensity. 140 dB for a brief second, such as a gunshot as close range, certainly can cause damage, as can say 85dB for 8 hours, or 88 dB for 4 hours, 91 dB for 2 hours etc. Essentially doubling the dB (which is a log scale) halves the safe exposure time.
95dB from the IEM's is running them pretty hot, but per that as an example, it would still be likely "less bad" wearing those for a one hour set, than spending one hour in front of an amp at 120 dB (which is quite bad). Thing is though, that one might not stand in front of the amp for the full hour, so if your off side stage a bit, maybe your only getting 90dB there, while the IEM's are potentially going to deliver 95 dB near the whole time.... gets to be some complicated math.
Realistically though, no one needs to be running 120 dB in most venues, its going to blend into vocal mics, blast audience members too close to the stage etc. Really the point of IEM's is that you can turn the stage volume down, as people don't need to turn up to "be heard", and still hear yourself anywhere on stage, at a volume that is loud and clear, but ideally 88 dB or so less, just to stay in the "safe" range.
To further compound things, IEM's, or custom shell hearing aids for that matter, really don't attenuate sound very well. They really don't provide much of an ear plug/hearing protection effect on their own. No where near 25 dB attenuation, and its going to vary on the frequency as well. If one guy runs IEM's and the rest of the band has loud amps, loud wedges etc, its a difficult situation.
Overall though, as I've worked with lots of guys making impressions, and also have engineered, as well as being a musicians myself, I find that once groups go IEM, the overall stage volumes go down, they get a very good clear signal, assuming you've got some nice multi-driver IEM's (not cheap) and singers, guitarist etc are amazing at how well they can hear themselves, and whatever other mix they want, without needing a ton of volume, which is usually used as a crutch for lack of clarity in hearing oneself.
IEM's are great though overall, and when used properly, along with a quieter stage, certainly can help conserve musician's hearing, while often providing even better performances
. But volume needn't be one of them. A single 12" speaker with four times the throw of a guitar speaker can move just as much air as 4 guitar speakers. But yes, there are other differences.
