I shoot Trap, Skeet and Sporting Clays quite a bit and I have never seen much of a forward lead allowance change by changing the speed of the shot.
For me if a 40 yard crossing shot needs a medium sized lead like 5 feet or 8 feet with a 1200 fps load it still needs a medium sized lead to hit the target with a 1300 fps load.
On huge leads like 10 feet and longer at over 40 yards a 100 fps or 200 fps faster or slower load is not going to make it a medium or small lead, it will still need to be a huge lead.
1450 fps steel #7 shot takes 0.147 sec to get to 40 yards, speed at 40 yards is 546 fps
1450 fps steel #3 shot takes 0.129 sec to get to 40 yards, speed at 40 yards is 686 fps
1450 fps steel #BB shot takes 0.119 sec to get to 40 yards, speed at 40 yards is 779 fps
1220 fps lead #7-1/2 shot takes 0.148 sec to get to 40 yards, speed at 40 yards is 602 fps
1220 fps lead #5 shot takes 0.137 sec to get to 40 yards, speed at 40 yards is 686 fps
The difference between Steel BB shot and Steel #7 shot at 1450 fps is 0.028 seconds at 40 yards. In 0.028 seconds a crossing 40 mph target can move 19.8". If you need a 7 ft to 10 ft lead on a 40 mph crossing target at 40 yards a 19.8" lead difference is not that much.
Look at the time it takes the 1450 fps Steel #7 shot and the 1220 fps lead #7-1/2 shot to get to 40 yards. .001 seconds isn't a very long time.
The Difference between Steel #3 shot and Steel #7 shot at 1450 fps is 0.018 seconds at 40 yards. In 0.018 seconds a crossing 40 mph target can move 12.7 ". If the shots are quartering away or inward, the lead difference will only be about 10" and 6".
One of the Sporting Clays instructional videos I watched said to break your leads down to small, medium, large and extra large. They didn't want you thinking about feet and inches so much. So maybe a small lead would be what you think is 1 to 3 feet, a medium lead would be 4 to 8 ft, a large lead 9 ft to 14 ft and a extra large lead would be over what you think is to long of a shot. I only use a extra long leads on Sporting Clays Targets not birds.
I think way to many shotgun shooters try to add 6" to a foot to a short lead when they just missed instead of going to a medium lead.
How many shooters can tell a target is crossing 40 yards away and know exactly how many feet they need to lead the target?
From what I have seen and what works for me is, I don't worry about 6" to 18" of lead if I miss a target at 40 yards and under. I just double the lead and shoot again and that has worked for me most of the time.
If you are missing crossing targets at over 40 yards away, can you tell you just lead that target 10 ft and that you really need a 12 ft lead? If you go from a medium lead to a large lead that is better than trying to add 6" or 18" at a time. Most of the misses at 30 yards and farther are missed by feet not inches and since your pattern is close to 30" wide you might as well make a good sized adjustment not a 6" adjustment.
KPY shows that a 1400 fps #9 lead pellet takes 0.144 seconds to go 40 yards and it shows a 1250 fps #7-1/2 lead pellet takes 0.145 seconds to get to 40 yards.