Any choke is fine for buckshot. How it performs with your barrel and choke you will have to determine. The reason they do not recommend slugs is that the barrel is overbored and accuracy would be lousy.
The 835 tubes are "overbored" to reduce recoil and improve patterns. That plays hob with big chunks of lead (lugs) bouncing down the tube. They do make a special slug barrel for the 835 though.