Hey Ian,
I haven't tried the Io one, but I have used the black magic intensity with a mac mini. It was running off 1 SSD, with 16GB ram and it worked pretty well. It still "looks" like a capture card but I was pretty impressed. Not much latency at all, mostly just looks a little chunky with fast action if that makes sense.
I tried it with analog, and HDMI input, as it did not have SDI.
The biggest issue is you have to specify *exactly* what your input source size and refresh rate are.

I really wanted to try and use it on a dual output but Mac minis only have 1 thunderbolt port and the black magic ones don't have a pass thru, so I couldn't do dual output... Also triple head2gos seem to not work on the HDMI out of the mac mini so I couldn't cheat there either. No such thing as a thunderbolt hub yet either...