Is it the 1/3 scale or the 1/4 at 110 inch wing? I ask because you mention a 10 foot wing and the 1/3 scale on his web site is 12 foot while the 1/4 is just over 9 foot. What does it weigh now? Mick shows the 1/3rd coming in around 22-24 pounds and the 1/4 at 10. Might e-mail Mick and ask him. Go through the contacts page on his web site. He responds in a day or two.
Mick Reeves

And what kind of wind conditions are you talking about? What wind speed? Steady? Gusty? At the ground or higher up?

Adding weight to a glider/sailplane is a common practice. It improves penetration especially in rough air. Real and model both do it all the time. The full size competition guys use water ballast as do many larger models. I have a Lengionair 140 (from the 70's), 140 inch wing, which is designed with aluminum tube spars intended to carry extra weight in the form of .44 / .45 caliber lead slugs. It shows that on the plans and in the instructions.

As mentioned, you might find a need for a little higher flight speed. But being a Motor Glider, that should pose no problem. I do know Mick has really put the design through its paces doing hard loops and rolls with it so did you build it according to his plans? Keep in mind that a lump of added weight directly on the CG will focus more stress to that part of the structure. Just make sure the air frame is up to it. And it will float forever on final if your idle is set too high. But yours being electric shouldn't have that problem.

Depending on the scale you have, I'd say add some weight and see how it goes. Just a bit to start with. Then work up to what suits you. But again, contact Mick. I have no doubt he will respond with some interesting info on the design.