The modern neoprene materials are another factor. 20+ years ago the neoprene was about as conforming as a cardboard box. In the past 10 years or so the materials have improved. Fit is so much better. This does wonders for water flush through. There is a lot more emphasis on water control and not so much an shear bulk for staying warm. If a wetsuit doesn't fit right it doesn't control water flow and that means it doesn't matter how thick it is.
I second this wholeheartedly.
My first cooler weather diving happened about 15 years ago when I started assisting classes in Colorado. We wore the shop's 7mm Farmer Johns, meaning 14mm on the core. I was a little chilled in our cool (not really cold--58°-60°) water. A couple years later the shop got rid of them and went to a plain 7mm suit made of newer, stretchier material that fit much better. I thought they were noticeably warmer, and so did the rest of the staff. They were a heck of a lot more convenient to use.
I have dived with a drysuit in those waters for a decade now, but I had a recent disaster in which the suit sprung a leak on a teaching weekend, and I borrowed a 7mm suit from someone. It was a couple years old, and I thought it was better than the ones mentioned above. I was comfortable with the diving I did in it, although I did not do any 2 hour dives. As soon as I got home, I ordered a new 7mm suit of my own so I have a backup, and I think it is better than the one I borrowed. I might be willing to try it for a longer dive.
All that means a new, good quality 7mm suit that fits you well will do a better job than the old, clunky 2-piece suits. That is not that much of an endorsement, so I will go a step further. My first wetsuit used for general diving was a 3mm Farmer John. I wore that for a while until it mysteriously shrunk
, at which point I bought a standard 3mm suit. I thought it was warmer than the 3mm Farmer John as well. It is hard to believe that the better fit makes all that much difference, but it sure seems to. So, in general, I am not a fan of 2-piece suits.