Bottom line on this is that you can only input 1920x1080 into catalyst at this point.

I have tried the following: Aja IO 4K, Active Silicon Cards in a Sonnet case, Blackmagic UltraStudio 4K, and Blackmagic UltraStudio Express.

All of them work just fine. I cannot comment on latency as i never really looked at them side by side.

Active Silicon Cards: Richard has invested a lot of time making them work and they do just that work. Every Time!
Aja IO 4K: works, flexable, but you only get access to one input at a time. Also that is not switchable via catalyst. Maybe you could script that?
Blackmagic UltraStudio 4K: Expensive and very picky on what signals it likes. For example it wants a non standard version of 3G SDI. Fail.
Blackmagic UltraStudio Express: Great for the price and you can get 2 to work on the same machine at the same time. Downside is no thunderbolt feed through.

Hope that helps!

I am now using the Active Silicon Cards and sometimes the Blackmagic UltraStudio Express depending on the project.