The decklink extreme, I believe, is an SD only card. It won’t handle HDV, unless you plan to edit your HDV in SD.
I have a Kona 2 (not LH) and it works fantastically for my purposes. I tried decklink both SD and HD cards, and the Kona 2 has been far less problematic, but I’m just one guy with one opinion. If you are planning on downconverting your material, double check decklink’s settings and I’m not sure they can handle downconversion in hardware just yet. I know for a fact that 720p24 downconversion used to not be available at all on any of their cards, but maybe that has changed.
Jeremy