http://bikecalculator.com/veloUS.html
Here's some purely academic calculations based on myself (360) and my target weight (200), with a 30 pound bike, and 150 watts average.
Code:
Wind Grade Myself My Future Self Difference % difference
0 0 15.81 17.13 1.32 8%
0 1 10.32 13.47 3.15 31%
0 2 6.99 10.43 3.44 49%
0 3 5.15 8.20 3.05 59%
0 4 4.06 6.64 2.58 64%
0 5 3.33 5.53 2.20 66%
0 15 1.19 2.01 0.82 69%
5 0 13.27 14.32 1.05 8%
5 1 9.08 11.47 2.39 26%
10 0 11.01 11.81 0.80 7%
10 2 5.81 7.91 2.10 36%
0 -1 22.34 21.01 -1.33 -6%
0 -2 28.45 24.77 -3.68 -13%
0 -3 33.87 28.30 -5.57 -16%
The first thing I notice is that based on weight alone, with no grade and no wind, I can go 8% faster if I drop 160 pounds. That's not what I would consider especially significant, but 1.32mph is 1.32mph. That'll cut 6 minutes off my morning commute... assuming it's all flat. But it's not, there's a lot of small little inclines of 1-2% grade.
The noticeable thing is how much speed drops off as soon as a grade is introduced. The tiniest amount of hill suddenly increases that 8% gap to a whopping 31% gap. That's much more significant. Now we're looking at a 27+ minute difference in my morning commute. The higher the grade gets, the more the gap increases too.
This is why there are weight weenies. To a pro racer, who goes up 15% inclines on Le Alpe D'Huez, even a single pound makes a world of difference in speed. When we're talking 100 miles in a stage, it adds up. As Depeche Mode once said, "Everything counts in large amounts".
Sample calculation. 130 pound rider, with a 15lb vs 16lb bike. 20 miles at 8% grade. That's a tough mountain right? But something that the pros probably do. 15 pound bike goes 5.67mph. 16 pound bike goes 5.63mph. Tiny tiny difference. But... over 20 miles, 15lb bike finishes in 211.79 minutes. 16lb bike finishes in 213.14 minutes... a minute and a half sooner. 15 pound bike wins.
Now here's where the fun begins. Look at the negative grades. As clydes, we get a big advantage over non-clydes when we're descending. Not nearly equal to the advantage that they get when climbing, but on a -3% descent, they actually go 16% slower than we do. This is why, in this years Tour de France, Thor Hushovd was able to win 2 mountain stages... they didn't end at a peak. Those stages ended at the foot of a mountain. There was no way he was going to win going up the hill, but when it came time to go down, he flew past everyone, despite weighing more than them.
Wind doesn't seem to make much of a difference if weight is the variable. In fact from the calculations the percentage difference actually goes down if there's more wind. I'm not sure how they figure that. I'm not certain that they factor in the fact that as a clyde, we simply have larger frames and therefore are bigger wind-catchers.
So basically, yes. Weight loss will help your speed. Since the speed loss from climbing is more than the speed gain from descending, your overall speed will go up. It's a matter of physics, really. But training will also help.