Batson Enterprises has a weighted butt system. There are three different weights 2oz, 3.75oz and 5oz. It consitst of a threaded butt cap and the individually available screw on weights. I have a series of Lamiglas rods I'm building that all have the 5oz weights on them.
Fuji/Matagi also make an adjustable weight system - not as heavy as the Forecast.
All things considered adding weight to the rod generally decreases sensitivity, however it will balance out the rod especially for your spinners and what not[/QUOTE
When I was in the tackle business, my partner and I experimented with several rods from Gregg to Loomis and discovered that balancing a rod actually increases sensitivity. As near as we could figure, changing the fulcrum of what is essentially a lever in one's hand from a spot toward the tip of the rod to the reel or the center of the hand itself allows the tip of the rod to better transmit input. Or something like that. Regardless, it seems to have this effect and if it is subjective, there were quite a bunch of avid fisherdudes who agreed to a man it was true.
In addition (and perhaps even more surprising) it reduces fatigue. It was for the latter reason that one of my early experiments was on a 9-foot Kalin (garage find) which served as a bobbe rod for Chinook fishing. I used a longish butt cap in which I could place a couple lead disks. It became a pleasure to use. Many put quarters inside oversized butt caps on lighter rods. We started fooling around with trying different methods over 30 years ago.
On the custom rods we built at the store, we'd balance the rod before finishing and gluing the appropriate amount of weight inside the butt section of the blank. I s'pose it's worth a mention that these were all built to order for individuals, not off the rack.
This would certainly be an easy thing to try (without permanently altering a rod) to see if others share an improvement in performance.