The only mod I've made to a Dillon myself is I turned and bored my own powder drop/actuator tube. I wanted the Lyman M style neck flare with a step in it. I learned later that Dillon offered that option at one time, but dropped it. Probably because the step has a tendency to grab a little at the brass, so it doesn't feel quite as smooth, but treating it with Moly Fusion fixed that. I also didn't yet have a toolpost grinder for the lathe at that time, and now I might just put a step in the provided tube that way.
The difference is the bullet sitting upright in that step starts in straighter. With the original on my Square Deal I'd get a number of rounds where the bullet base position bulged only one side of the case due to bullet tilt. With the step I never see that. The base is always mirrored all the way around. It isn't always quite perfectly uniform because case wall thickness isn't always quite uniform, but it is way better.
Demolition Man,
Thanks for feeding back. It adds to everyone's knowledge base.
On the primers, when I got my Square Deal, which would have been the late 80's, it had a lot of trouble with CCI primers. That was before CCI revamped their assembly line and process ('92, IIRC). The old CCI primers had burrs on the edges of the cups and they all felt very hard to seat, and I got a lot of high primers from them where Federal, Remington, and Winchester would all slide right in. After the revamp, CCI burrs were gone and they started seating nicely and still do.
More recently, I've noticed Russian primers have burrs on their primers. I bought 10,000 Tula primers at one point because they produce such remarkably low velocity SD's, but the seating has been no fun. I generally have to run all brass, even new brass without crimps, through my Dillon primer pocket swager before the Tula primers will seat reasonably easily. But once you get them in place, they work well. I haven't tried the S&B's myself, but suspect that's what you are seeing with them.