The answer to your questions is absolutely yes.
I just made the switch recently so my experience is limited but here is what I noticed. Since weight makes a difference, I weigh about 225lbs. In a 7 mil suit diving AL80’s I would dive with 22 lbs. Which is most likely two or more LBS. over what I needed but its what i felt comfortable with. With a Steel 120 I was able to shave off 6 lbs and most likely could have gotten away with 8.
As far as bottom time think about it in cubic ft. A 120 is exactly 1.5 times the cubic ft of an 80. I could be wrong but my guess would be that steel 120s are High Pressure. Since more pressure means more air you will have slightly more than 1.5 times the amount of air you would have when diving an 80 filled to 3000psi.
Good luck, dive safe,