I have a 120g 4x2x2 rimless with 3/8" glass and no bow whatsoever. I have about 6500+ gph (3 vortech's and return of 1200gph) flowing in my tank and nothing is wrong. I would send that tank back.
i don't think 1/16" is a lot imo. thats 1/32 each panel, about .032 of an inch about the thickness of a cigarette box. to measure in 32nds accurately you don't use a tape measure.
rigid material is going to flex a certain amount imo. the longer the piece the more the flex.
I think 1/16th of a bow on a rimless tank is not something unusual. I have a 100g 4x2x20"H and it bows about 1/16th on the top. That's with 1/2" glass. Glass is a quite a flexible material and as soon as it's glued using a quality rtv silicone, it will hold. I'm no expert though, you should ask someone who build aquariums for a living to get the best opinion.