Schedule 40 is a measurement, not the pipe's quality. All sch.40 pipe will have the same i.d. and o.d. which means the wall thickness will be the same. That has nothing to do with HOW the pipe is made, i.e. solid core or foam core. We do not use PVC drain lines here, but where it is used, foam core seems to be the material of choice for drain lines. But it is not a "pressure" material so it is not used for supply water lines. If it were a problem for you, your specification should have called for scheduler 40 pressure rated piping.