This is my point - the algorithm challenge is to do a given compression task quickly, and Shannon has no limits I'm aware of on how fast that can be done. So there's no problem of approaching physical limits the way there would be for, say, "Encode this file in the smallest space possible".

Edit: The stuff below the line either wasn't there when I replied or I didn't see it. You're right though, if linear-time implementations already exist, then you're basically hard-bounded.