I have seen plenty of images printed larger than 16x20 from 35mm negatives. Recently, I was at a local museum and it was showing images from 70s. It was obviously from 35mm because some of the images showed film rebates. None of them were grainless images as should NOT be but wonderful images nevertheless. They were subject appropriate, mood appropriate, and image appropriate.

If your buyer is used to plastic smooth images, he/she might have some objections. Recently, I've been finding out that technical quality of images are largely for photographers, not buyers. Why not make an 8x10 print of an important part of the print at the 16x20 magnification, NOT pointing out the grain-ness but send it to the buyer and see what the buyer says?