Affect of 15% weight reduction on speed

Hi all - I have an odd post for you to ponder :) Recently I've decided enough is enough and it's time to shift some unwanted poundage. Over the past few months through exercise and eating much better I've dropped about 15% of my original body weight, going from 207lbs to 175lbs. At the same time I've been swimming a bit and making an effort to keep on some muscle too. I know it's a very very hard question to answer but am looking for people's estimates on what this sort of weight loss would do to your swimming time over longer distances if I was able to keep everything else static (stroke, flexibility, strengtht etc). The only difference if possible would be there would be less body weight, and hopefully a better shape for moving through the water. I know that due to water being denser it's not as easy to say as it would be in relation to running etc, but say over a 5km open water swim, what would people guess the % improvement would be as a result of this? Cheers GC