I know that carrying more guns around increases the cooldown times on my skills, but what is the exact relationship? Does it influence every skill the exact same way or are some more susceptible to this than others? Is it percentage based or a flat increase in the number of seconds I have to wait? Is there a maximum amount of penalty I can have, and if so what is that?
The cooldowns of Shepard's powers get a "weight modifier" based on how encumbered Shep is. This is a strict percentile, applied the same way that the cooldown reductions along the skill trees function.
(That is to say, 100% cooldown reduction = you can use the ability twice as often. If it helps, think of it as "Uses per second". A 100% increase in uses per second means half the original cooldown).
The bar itself goes from +200% to -200%, depending on what percentage of the weight capacity is used.