When looking at the Ikea bookcases, we looked at the specs. Under the tab 'product information'. The one that's 11" deep is able to hold only 66 lbs. vs 93 lbs for the one 15 3/8" deep.
We weighed the sefarim of a pile 30" wide. (The width of the shelf.) It's weight was too much for the 11" shelf, so we got the deeper one. So far, happy. 2 years with the 'mesivta', no warping.