8 1/2's are great warm weather loads but will only dust targets in below o weather here in Minnesota. I personally use a 3 dram load of 7 1/2's when the Tundra is frozen...Use the 8 1/2's down South or in warm climes and you will love them. lol Dale
There is the same difference between 8 and 8.5 shot as there is between 7.5 and 8 shot. Bags of some brands of shot and shells from some manufacturers that are labeled #8 shot contain shot that can range from 7.5 to 8.5.
Seeing the target well and keeping your head down is very important. Shot size is of little importance. But, we often emphasize the minor points and omit the major points.
sshotshell- My direct answer to your question is that perhaps the top 20 shooters in the country could tell a slight difference between 8 and 8.5 shot from the 16, but the rest of us would never know the difference.
For the 12 not much difference. When you start dropping down in gauge you notice that some guns and chokes prefer some shot sizes over others. Even more noticeable when shooting long range sporting clays shots and open choked short range shots in skeet and sporting clays. More than once with my 20 I have gone from several misses in a row to several hits in a row by doing nothing but changing shot size on long range shots. I have seen the same thing with the 20 with skeet chokes. A couple of misses do to a thin pattern with 7 1/2 shot go to 25 straights with the same choke and no. 9's.
Back when I was in the Air force, I shot both trap and skeet a lot. I did not like changing loads a lot and loaded 8-1/2s for all my 16 yd. trap loads as well as my skeet loads in the 12/20/28 ga. They worked fine for all of it and the targets never knew the difference. One day I was at a trap shoot and grabbed 4 boxes of shells. I broke a 95(including one 25 straight) from the 22 yd. line(my yardage at that time) and omly discovered I had been shooting my 8-1/2 loads after I was done.
Shooting trap at 16 yards with a 12 gauge and a full choke I doubt that it does make any difference what shot size you use. Any of them will smash a target. 8 1/2's are an excellent choice for someone wanting 1 shot size for trap and skeet.