It depends on how you're judging performance. We can quantify to some degree based off numbers for mass, speed of travel, type of terrain, nutritional intake, caloric expenditure, temperature, distance covered and any other number of factors you can find a number for and make a hugely complex or simple model to measure the performance gain. I like numbers, but they only get you so far.
Humans are not designed to be fast, which is the normal criteria most people think of when it comes to hiking. We are designed for endurance. If you want to be faster, you need to train your body similar to any other athlete, and walking inherently tops out around 4-5mph before you have to switch to running/jogging gait. Andrew Skurka can do 40+ miles a day, but he also is an ultra-marathoner with the associated training. AND he hikes up to 16 hours a day (16hr X 3mph = 48miles).
Really it comes down to how you feel. When you did a 16 mile overnighter before losing the various "weight" how did you feel at the end physically? Were you tired or exhausted? Sore muscles? How long did you have to spend hiking to do such a trip?
Now look at the same trip post weight loss, I'm positive (barring injury) that you feel better, more refreshed than before. So your energy levels are higher now, not because you have more inherently (with a gut, you technically have a larger store of energy to draw from than without) but because you use up a lot less. You're mind is also probably more alert and less exhausted from having to force yourself to plod along.
I guess really what I'm saying is that there are a multitude of factors, what do you want to see as an "improvement"?
EDIT: The posts above me better convey what I was trying to say in mine. One thing I would add is that going Lightweight or UL is more of a philosophy. It slipstreams your ability to get outside. For the physical benefits, they only truly shine when you start covering long distances, whether it be over a day or over a few months.