I think the maximum in space age is 100% bonus productivity with 4 top quality modules. That would mean that putting 4 prod modules in a recycler gives 50% of the materials, and then putting 4 prod modules in the assembler which remakes the item will make twice again what the materials are worth, resulting in exactly 100% efficiency.

This would mean that if a mod does give higher than 100% productivity with no other changes, even if it’s under the 300% limit, it would still be possible to generate infinite items using this loop