As stated in an earlier post, I set my down thrust using the amount molded in the cowl. However, I don't think you'll be able to solve the cg issue with add'l down thrust. The manual places the cg 130 mm from the leading edge at the fuse. I've learned that you can't trust the manuals on these foreign ARF's.
Did you move the battery fwd? What battery are you using. Mine weighs 509 grams (6S 3000mAH).