I've shot a lot of #9 for trap, as mentioned they work well enough at the 16 yard line and for the first shot in doubles providing you don't ride the birds too far out. I found a tight choke(Improved Modified or Full) helped as it puts more pellets on the target which can be important with such small shot size.
I often shot #9 with 7/8 oz of shot whether in a 12 ga or a 20 ga so I wasn't using much more than you will find with a 28 ga. When I didn't use one of the larger bores I sometimes used a 28 ga but usually used a 410 which has even less shot. I don't ride the birds much and tend to take the birds pretty close to the house so the pellets tend to have enough energy to cleanly break birds. If he tends to let the birds get out a bit the odds of not breaking a bird go up and it can be an incentive to take the birds a bit quicker. Learning proper hold points is very important as it allows one to see and intercept the target much quicker.
