In my experience ,pigeons have loads of down feathers which is quite thick,on the breast area, 7s would just bounce of them,you need something that would penetrate that area.
what you say "I use 7's for decoying, if your decoying and your decoy pattern is pulling birds into a predictable killing area then 8's.. even 9's could be used to good effect and good for building confidance in young shots."
The pigeons in my area would just laugh at 8 or 9s,that would be like throwing a handfull of smarties at them
6s all day will do the job,but thats my personal opinion at over 25 yrs shooting