I haven't tried it yet, but I'm about to start playing.
I'll report back with my personal experience.
UPDATE: Ok, so I've played through the starting areas of Downpour and have just arrived into town, which is where the framerate/lag problems were worst for me (in the streets).
I can say that the patch has helped. In my three prior playthrough's of Downpour un-patched, the game would consistently lag outdoors on the streets if I was moving.
With it patched, I ran around a few blocks and through some backyards as well as fought a couple monsters & stuck around for a storm; and it did almost no lagging. Yay!
Before the lagging severity was a 8/10 for me, now it's a 3/10.
So no, the patch didn't wipe away the lag completely. The game will freeze up for a split-second during an autosave, & it still lags occasionally, albeit very rarely in comparison to the un-patched game.
As of now I'm happy about it. I hope it retains this level smoothness throughout.