I think I know what the OP is looking for, and I can relate. A couple years ago I was asking myself the same questions. I'm not uncommonly heavy (~200lbs) and of fairly typical proportions, but I just couldn't find the bike that seemed to fit right, ride right, and have the overall performance I wanted. My story ended up being a really bad experience with (so far) a happy ending.
I was riding a production steel frame for years, a Gunnar, that I really liked the ride quality of but it didn't have quite the performance I desired and a couple of the fitting adjustments were at their limit. Originally I was going to order a custom steel frame, but then I decided that if I was going to spend that kind of money then I wanted the durability of titanium. So I visited the shop of a custom ti builder in Colorado (not Moots, but in the same town as Moots...that's a hint), described to the builder in excruciating detail how important good ride quality was to me, and a couple months later I had my new ti frame.
It was horrible. The fit was spot on, which convinced me that going custom is the right thing to do for the discerning cyclist, but this particular frame had one of the most brutal rides of any bike I've ever been on. The build kit was off my old steel frame so I knew that wasn't the problem, it was the frame that was at fault. Despite many guarantees about my satisfaction prior to ordering the frame, the builder refused to do anything for me. Since the frame was worthless for longer rides, I had no reason to keep it so I got rid of it at a significant loss. I had a guarantee in writing, but taking the guy to court would have cost me more in the end.
So that was my bad experience. (Note: I once started a thread on BF about all this but since it was critical of a frame builder the moderator deleted the thread against my wishes, therefore I won’t name the builder here. PM me if you want to know it.)
Still needing a frame, I decided to go back to steel. I took the fit from the custom ti frame and had a Steel Crown Jewel built for me by Independent Fabrication. It’s is the most wonderful bike I’ve ever been on, and I’ve owned at least 20 different road frames over the years. I knew it would fit better than my Gunnar (it was the same fit as the failed ti frame after all) but I wasn’t expecting how much better the performance would be. I didn’t give up any ride quality and I got a better fitting and better performing bike out of the deal. I should have gone with IF in the first place.
My next frame (there’s always a “next bike” in my world) will most likely be an IF Ti Crown Jewel, but it’ll be hard to beat this Steel Crown Jewel. And I’ll never get rid of this frame.
So that’s how I’ve gone through both the good and the bad when it comes to custom frames.
Bottom line: there’s much more to a custom frame than just making it fit. A good builder can make the bike ride how you like, handle the way you like, and perform the way you like, or at least strike a good balance between all these competing demands. If the guy knows what he’s doing, it can fit you better than you ever thought possible yet still have adjustability to spare to allow the frame to adjust to you as you age. A good custom frame can be the last frame you’ll ever “need” to buy, and in that respect a custom frame can save you money in the long run.
This is just the short version of my experiences with custom frames. PM me if you have any questions.