It is funny that this thread came up. When I was visiting my local cue maker last weekend he was working on 2 brand new OB Classic shafts that the customer wanted to be taped inside the joint area so that 2 counter-sync weights could be added/removed. I found it odd but this customer has this done regularly and loves it.
I personally love heavy shafts and my current playing shaft is a Predator 314-2 that was made from a Partial and weighs 4.3 oz.
Well, it would sure change the balance of the cue, and I since quite a bit of your game can come from the feel and confidence in your stroke and equipment, I can see how a cue that you like the balance of will help with a shot. It may not actually help it as better equipment, but much of a pool players equipment resided in your head.
It's like when people ask why would you get a $1,000 cue when a $300 cue could be just as well made. It comes down to how your are thinking about the equipment and I guess the euphoria (for lack of another work I can come up with) you get from playing with a well made and desirable cue.
I tend to play oddly well when I borrowed someone cue to try that I liked the feel of, or on a new table that I liked the speed and setup of. Once time I went to a new room for a tournament (Amazin Billiards in MA), never been there before. Got on a practice table with a guy, and promptly broke and ran 2 racks, something I very rarely do, I'm pretty sure it was because the table was setup very well to how I play for speed of the rails/cloth and how well it rolled.