We’ve done the same thing on a green screen studio shoot, used a Mac with an Xserve RAID and a Blackmagic card, went straight in from the camera head via HDSDI, we also recorded to tape as well as a backup.
Quality is very good and really easy to pull keys from. As a comparison we captured some of the material from the HDCam tapes and there was a visible difference in quality as well as ease of keying.