40lbs doesn't sound like too very much. From what I understand, you'll get spidercracks in the gel-coat where the tower mounts to the boat. The spidercracks are a realitivly easy thing to fix.
On my tower, I'm running 2 Rubicon 450s @ 88lbs, 4 Skylon Bat Racks, one Skylon Light bar. I believe the weight that the boat sees to be somewhere in the neighborhood of 135lbs to 190lbs loaded with boards. I hate rolling double-ups for this reason.