Would be nice to hear someone who runs one answer, but here's what I've picked up. I believe Jody from ARC said (don't quote me) in one of his videos that the 390 carb worked good on methanol but he wasn't able to get one to work as well on gas. As far as venturi size you're going from something like 17mm on a bored 196 to 21mm on a stock 390. I've seen 390's that are bored to 24mm for sale online. I'd say that's a substantial move up in size.
I have only seen 390 carbs on the 212s in alcohol applications. Infact we have one on a predator 212 on alcohol with a 390 and it works good. I don't know how it compares to bored stock carb thought cuz I have never ran one. I saw with the right setup and cam you can run a 390 alcohol carb on a 212 and it will run like a beast.
The 390 carb will flow a considerable amount more than the bored stock one. With a good head the 390 carb makes far more power. I run stocked bored carbs with 28.5 valve heads and 390's on 32 valve heads.
I run alky in 2 built 212's. Started out with out of the box NR 390 carbs and they both ran very well. Recently switched one of them to the 24mm version and it made a significant improvements. Both motors have 32mm intakes and both run at 8500 rpm. I have a 28mm Mikuni slide on and animal and while its a really good carb I personally woudnt spend the $$$ to put one on my 212's. I don't think it would run that much better than the 390 carb.