i'm not sure on how the agility stat calculates as there is no way to see the numbers per rank (like strength with carrying capacity).

does it calculate by 10 per rank??

does 10% movement speed mean adding 10% of current total agility special stat??

(if i have an agility stat of 11, would it be 11 + 10% of 11??)

Not sure either, but I was lucky enough to get both and able to see a big difference while sprinting. This was with Agility 9, 2 ranks in Moving Target and 6 Endurance. I think that the Sprinters work like Travel Light in New Vegas. Endurance and Agility combined plus Moving Target help to consume less AP while sprinting, while Sprinters just allow to move faster. I cannot isolate the increase in speed because I already have Moving Target in the equation. It would be a good idea to test it with a very low Agility and Endurance character and see if it makes a difference.