I'm very pleased with the feel of this set up. The weight of the new balls is so close to the originals that I can't tell the difference, so that hasn't changed.
The 25 year old trackballs have been through a lot. I've found a lot of (what I have euphemistically named) "bar wax" inside this machine, and the trackballs were pretty gunked up. The functioning trackball is very smooth now that I've thoroughly washed and re-greased it. The bearings didn't need replacing, just greasing, and the shafts of the rollers are steel and straight, so there isn't much reason to replace them.
The original balls had been waxed a number of times, I guess in an attempt to shine them, but the actual effect was to gunk up the mechanism and make the balls roll jerkily. I put the old balls in my dish washer, and they came out nice and smooth, and it improved their feel immensely. (Please never quote this paragraph out of context.)
These new balls are (as advertised) so shiny! But they are also well made as candle pin bowling balls; they aren't so slick that you will lose grip on them. The plastic they are made of is, as I've mentioned, very much like a regular bowling ball. These have the same "touch" as a bowling ball. It's not unlike glass. The combination of the smooth rolling and the "touch" of the new balls has marginally improved the feel to me.
The new balls are so similar to the originals, except in appearance, that I would call this a mostly cosmetic modification. I'm at the point in this project where it makes me feel good to spend $50 on making the trackballs prettier. That may be an indication that it's time to move on to another cab (or a less expensive hobby

).
If you had pits or scrapes or other deep defects in the original balls, this would be a more-than-cosmetic swap. The new balls work as well as, or better than, the originals.