What makes you think that there is a quick way to remove weight from Zeta? Zeta's "weight problem" (it's really not heavy at all for its size and price range -- full size 4 doors with a 350+hp V8 all weigh in the neighborhood of 4000 pounds) has been known since before it was introduced in 2006 in Australia. If it were going to get lighter, it would have already.
I can't see any significant future weight cuts in Zeta (I'd guess max of 100 pounds), other than going to 4cyl engines. Fuel economy improvements will mostly come from more efficient powertrains, I'll bet.