I hope that it works as well for you as it has for us. There are many benefits. Files do not sit on the shelf without being reviewed. Files do not get buried on people's desks and forgotten. Closed files do not sit on the shelf until we have no more space to deal with them. And another big benefit - since all files come back to the cabinet pretty much daily, I can always keep my filing up to date. No more two-inch thick piles of unfiled documents waiting to be filed because a lawyer has (or lost) a file. And of course keeping filing up to date helps the lawyers keep clean desks and makes it easier for them to deal with a file when it comes to their desk.
You are absolutely right about there being no difference between the files piled around your office and the ones in your file cabinet. The ones in your office are demanding all of your attention because you have not determined the next action and next date that you will work on the file. You don't go home feeling like you have cleared the decks because you are worried about forgetting about a file.
Keep visualizing what it will be like when your office is nice and clean, all filing is up to date, and all files have been assigned a date and next action. Imagine getting a call from a client who wants to know where you are on his file. You know where the file is, because it is on the shelf, not buried in your office. And when you pull it off of the shelf, you look at the front of it and you tell the client that you are waiting for xyz and that you will be following up on it on such and such a date. Depending on why he called you, you may need to move it up to today or tomorrow, but more than likely he will be happy to know that he is on your list and everything is under control.
Our file recall sheet has four columns: Date handled, Last action, Next Recall Date, Action on Recall. Pretty self-explanatory. Not only do you have a really quick way to see what you have scheduled for the future, but you also have a brief history on what has been happening on the file and the speed at which it is progressing. If the lawyer wants me to follow up on a file (report status to a client, for example - we are still waiting for xyz), he writes what he wants me to do in the "last action" file, highlights it in yellow, adds a new recall and next action, and leaves it for me to deal with. I can e-mail the client with the status of the file, rediarize the file, and put it back away.
Regarding bringing forward any file that you have received correspondence on - it may be beneficial to take a quick look through it before pulling all of the files. A lot of the correspondence will just be filed directly in the file, and reviewed in detail when the file is brought forward next. Quickly scribbling/stamping "file" or "recall" on each piece will save your assistant some time and reduce the number of files required to come to your desk.
It will take some discipline to keep this working for you and your assistant (and anyone else in the office who is involved). But you can do it! And I think you will find once it becomes routine that it will not take as long as you think to review the files that are brought forward each day. You are used to making a judgement every time you look around your office - that can wait, this is important, I have to do that today, that will have to wait until next week. Taking a second to write it down and put it into the system saves you having to re-think it every time you look around the room.
Good luck! You can do it! :P