Guess Agility is another illusion then.
All I really know is that increasing speed via GECK by .20 is more than enough to feel a lot faster. Even the .10 like the Perk does is quite noticeable. I didn't try to find out what speed would cause my pc to crash, haha.
But after testing that out a while back, I went back to default because of the balance thing. I didn't want
everything in the game to be running faster, and I didn't want only myself to be able to outrun everything faster.