In doing this is using an additional byte per user, per thread. Thus if 200 people at one time or another opened an account on this board, and the board reached 1000 threads (easily once in full production), this would increase storage use to 200,000 bytes.

This also assumes that the data is set up so that it would only be a matter of adding a new field, which I seriously doubt. So this would mean adding another table all together which is keyed to both the thread number and user key. This easily increases the size by a multiple of at least 5 (assuming thread and user keys being kept as short integers).

Then you also have to consider the extra time that will be required for this lookup.

It's fine for me as is, though it might speed up users with modems a bit.