On the C284E, C554e, & C754e the machine specs say that you can duplex up to Thick3 <256gsm, but I don't think I'd count on that for the C284e or the C554e. I should think that your 215gsm media would duplex alright on the C754e.

I hope that you're not going to try to run all this heavy paper through the booklet maker. There are very specific limitations on how many pages and of what weight that can be fed through the booklet maker. We see this all the time: Somebody buys a 25cpm office copier and expects it to become some sort of production print device.

=^..^=