Just curious here, but what makes you believe that there is room for improvement based upon 6 grams per 2.5 ounces?

I am really not in to BHO, so I don't have much advice, but I strongly recommend vacuum oven purging. Yes, they are expensive, like $1,000 an up!

The air bubbles in shatter probably contain butane.

There is one thing that I can advise that may help improve your product. Put your butane cans and your prepared tubes in the freezer for 48 hours before conducting your extraction process.