That's a really nice Manta. At 450 grams and she still glide at a very nice sink rate. I'm very surprised.
With some practice, I believe you can lauch a lot higher. Find a nice thermal day and you're very likely to find a thermal and go all the way up.
Light balsa is a must to reach 300g target weight. If you feel the wing strength is not enough, try using light balsa in the wing center and gradually move to the harder balsa near wing tip. Using light balsa for DBOX is probably one of the most rewarding area for weight saving.
As for Manta 2, I don't thing I can keep up with you learning curve.
Too many side projects on hand.
Max, check your mail box for printable plan.