Have I Hit The Point Of Diminishing Returns?

You hit the point of diminishing returns when you want to justify not spending any more money.

I don't think it has that much to do with money, but it certainly could end up that way. You are in a position now where you have your system tuned you your tastes and you enjoy it. I wouldn't think in terms of "better", but instead, think of "what would I have to do to get more enjoyment from my system". IMO, you would need to home demo components and cables individually to see if they really make you want to listen more or less. A supposedly better component might upset the balance of your system and diminish the listening experience for you. It would also be an idea to speak with a good dealer and let him make some recommendation based on your taste and existing system. A good dealer can do this - a box mover is just going to try to sell you what they want to move, so be selective. Good luck.

I wouldn't so lightly dismiss Cardas as "rolled off", as if that's a bad thing. From what I see, many, if not most of the complaints here concern bright or forward systems. Many audiophiles fall into the same trap - they construct these "accurate" systems and then bitch about the brightness. It's a delicate balancing act. The room has a lot to do with it as well. If you're in a big room and sitting pretty far from the speakers, a more neutral "better highs" cable may be the thing to balance out the degree of softening the room may have. But if you're sitting 8 feet away from the speakers, those cables/components may tear your head off. This is not a simple thing and you have to let your ears be the judge. There's an old saying - the more you open the window, the more garbage flies in. That's why I'm saying that upgrading has to be done carefully and preferably with the help of someone who knows the gear and whose ears you trust.

I think you need to isolate what you are trying to achieve. It's a small room, so you can only do so much. For example, one could be looking for more realism in the human voice, more bass impact, better sound at lower volume, etc. You can't just walk into this without a goal. In my experience, the biggest part of the puzzle is finding a pair of speakers that work in your room. When you find it, it's an amazing upgrade. The bad part is - IMO, this is nothing but trial and error and there's no way to predict the speaker that's going to work best. It's like the errors in the room combine with the errors in the speaker to somehow work out. My current speakers were fairly expensive (for me) and I wouldn't buy them without a home audition. Fortunately, it worked out great. But if you like those B&Ws, you could probably buy a used Harbeth SHL5 and sell them at minimal if any loss. I think that might be an alternative.