Have you tried this 550 card with MC, does it look good (is it deinterlaced?), and finally have you tried to compare it running in MC with the audio of a regular TV so you know if it's delay free with MC? If it is I have no problem buying it.
To answer your questions in order...
No. Not applicable since the first was no. And, no.
I use BeyondTV for
ALL of my TV watching and recording. With BTV and the ATI/Cyberlink decoder, and my X1900XT's hardware deinterlacing turned on, the output was beautiful (I say "was" because it's not hooked up to that computer and display anymore).
However, I don't have a TV directly hooked up to my DirecTV box, and I certainly don't care about the delay (I think it's been probably about 5 months since I actually watched anything "live" at home). I don't use MC because it can't control the channels on my external DirecTV receiver, and I don't have standard cable (and I'm out in the boonies in the US so I don't get good over-air reception at all without an expensive antenna). Even when I watch sports, I tend to pause the game for at least 15-20 minutes to give myself a good "commercial or half-time skipping buffer" (and the last time I think I watched a major sporting event at home was probably the World Cup -- I'm not your average American male).
That said... I can check the delay at least for you I think (it'd only take a coax cable I think) I'm not really able to check the deinterlacing anymore though. Since I reconfigured my living room, and my HTPC, the only display the computer that contains the 550 pro card is connected to anymore is a regular old interlaced TV (a big one, but an interlaced standard-def one). No progressive display at all connected, and it's gonna stay that way until I save my pennies enough to grab the Westinghouse 1080p LCD panel I have my eye on... It looks nice, but that doesn't really tell you anything!