I haven't seen anything that gives an actual numerical value to this...so...
1. what does each point of agility add to run speed?
2. what is the default value?
3. does travel light make a significant impact?
4. what is needed to outrun cazadores, or even deathclaws?
1, 2) I have not found the exact number, but here is another player's experiment:
Test method was two level 1 characters, one with 10 agility, one with 1 agility.
I started in front of victors shack, and walked down the road, across the road, and into the white fence of the house.
I made sure to aim at the exact same location. I did not turn my character at all, but just held down W.
I use a stop watch and started as soon as i hit W, and stopped as soon as i hit the fence.
It took me 27.69 second with the 10 agility character, and 27.72 seconds with the 1 agility character. The discrepancy is easily explained by my hitting the stop watch at slightly different times.
If it was even a 5% difference we would see over a second difference.
I did the same test while in sneak mode also, with the same result.
You could of course, question on his real life reflect consistency, but what we can get an impression on the subject matter is that, the difference is not much.
3) 10% while you wear Light Armour, and to intemperate that into gameplay flow, it is significant, traveling the lands and in combat, finding covers, pulls back just a little bit so your sniper can land the bullets in V.A.T.S. should you need to.
4) I can't tell you, but: "when you are ready, you don't need to".