How much would my marathon improve if I lost 10-15 pounds?

One rule of thumb is 2 seconds per mile for every pound of unnecessary/extra weight lost.

Another rule of thumb is one minute over the Marathon distance for every pound lost.

Jack Daniels has a formula which may give a little more benefit.

we have exactly the same height, current weight, and peak weight. "Ideal weight" is always a loaded question but it is probably safe to assume that the weight you've gained is not muscle. I think the right answer will depend a little on how fast you were at your peak and where you are at now. Presumably the weight loss would also come with additional mileage/training. My PR was 5:50/mile pace and now 40 years later I'm in about 9:00/mile pace shape. For me, if I could get back to my peak race weight I would think I'd be a full minute/mile faster if not a little more

Drop the 15 pounds and report back. I suspect a massive improvement.

Or maybe gain 15 pounds and see what happens?