Not all weight is created equal.
Let's say the 5'11" 180lb guy is 12% bodyfat, which if he runs and is lifting a lot that would be about normal. At 12% body fat he would have 158lbs of fat free mass (muscle, bone, tissue, water, etc). So having this guy lose 15lbs down to 165 would put him at about 4% body fat. The only people you'll see with true body fats in the 4% range are competitive bodybuilders during competiton. Given his muscle mass he couldn't sustain such a low body fat without losing fitness.
I don't buy the 2sec/mile/pound nonsense because not all weight is created equal.
Moral: Don't worry about weight. Worry more about body fat. Worry MOST about training.
Height/weight relationships really have little to do with running. You can find a 5'6' 120lb guy who can run worth shit, but you can find a 5'6' 145lb guy who can run well. It gets even more out of wack once you get taller.
With increased weight comes an increase in energy needed to move that weight. BUT, with increased muscle mass comes an increase in muscle power and strength which leads directly to an increase in running speed.
Ask yourself this question: Why aren't sprinters and long sprinters as tiny as distance runners? If weight and speed were inversely correlated you would expect the smaller sprinter to win correct?
In a nutshell, yes losing excess body fat will make you faster. Losing excess muscle, if you even can, will also make you faster.
Alan