It's not the upper temp limit its caused by the type of thermometer, IR therms can't get a good reading from shiny metal surfaces. Try sticking a white sticker (label) on the iron and test it then.
Mine is a real old iron and has the alloy anodising all burnished and matte on the bottom so I could get a pretty good reading. Temps tried from 120'C to 150'C, and i measured on the white paper on top of the film immediately after removing the iron. I was using small PCBs 2x3 inches etc. Part of my problem was technique with the iron, even with the bit of paper over the pnp with small PCB it was impossible to hold it, and moving the iron caused some smudging. Likewise over pressure caused some bleed, and trying to leave the rion stationary caused other problems because the iron surface was not at even temps, it was hotter in one place in the centre where I assume the element was riveted to the plate so that caused uneven cooking and the steam holes were tapered and these left cold spots unless I moved the iron.
Basically because the iron heats the entire film, the toner "wets" and the whole film can very easily move and smudge as you move the iron. I was also getting a measured shrinkage of 1% or so which is quite bad, and i assumed was due to overcooking, but running the iron cooler caused missed spots each time.
My theory on the laminator is that after the first pass the film stuck hard to the PCB, and was only "wet" in one place at any time (where it touches the hot rollers) while the rest of the film is much cooler and there was no shrinkage because the average heat x time is very low. Also the average PCB temp is MUCH lower, so the PCB is not expanding/contracting like a fully hot PCB under the iron which does a large expansion etc while the toner is wet.
Likewise there was no smudging or movement as the film is stuck to the PCB and all times and only one small line gets "wetted" at any one time.
Then add in the exact same heat on every part of the PCB and exact same temp on every part, with no exceptions, and the added bonus of much easier inspection after each pass in the laminator etc etc and just counting the number times means less error and stress.
That think that might cover some of the "physics" that duffy mentioned above.
I'm not saying anything bad about pressnpeel really, probably the opposite; with the laminator I think it gets close to the quality of home photographic etching but so much faster and easier.
And the last 2 boards I just left the pnp on the copper, and scratched off just the pads with a little screwdriver before I soldered. So it's a solder mask too although it's not pretty that way.