If you're willing to do cams, asymmetrical valve opening should have a noticeable impact on fuel economy at low RPM.
I'd argue pulling the balance shafts is a reasonable move too. It made my car fairly harsh on cold starts, but I also have solid poly engine mounts.
Any chance to get the EGR valve from the other Ecotec engines? If you're changing engine management, did the Cavalier variant have one?
Any chance of a top gear swap? That seems to make the largest difference.
EDIT: You could also K swap it.
Even the "baby" economy K20 is 180hp with a tune.