OutfitCharacterSpeed is the correct variable, it's just it doesn't seem to work as it used to do since 4.x - probably because the walk system was changed to allow you to specify if character should slide between positions or update position on each frame.
Switching between outfits or character animation index values is actually a better method to use anyway. It allows you to use different animations for each which is nice if you want to use an animation that actually looks like the character is running / jogging as opposed to simply playing a super charged version of the walk animation.
Instead of the OutfitSpeed, you could try adjusting the pause value of the walk animation. I don't know if it will work though.
local t = {"walk_e", "walk_se", "walk_s", "walk_sw", "walk_w", "walk_nw", "walk_n", "walk_ne"}
for i = 1, #t do
game.CurrentCharacter.CurrentOutfit.OutfitCharacterAnimations[i].AnimationPause = 40
end
I would recommend writing a function instead so you can access it much easier.