I tried Dolphin HD v5 and while it's better than the previous version it's still not as smooth as Boat Browser Mini when scrolling content heavy pages and there's no easy way to change User Agent or manage bookmarks among other things.
Boat Browser remains the king, well, 2nd best for smoothness next to Samsung's stock browser but 1st place for everything else!
All 3 browsers play 1080p flash hd videos as smooth as each other though.
You can change user agent on Dolphin 5 quite easily. Settings > dolphin settings > user agent.