I don't know about specific hp gain, but I was tired of dumping atf into a leaking rack, so I just cut the power steering belt until I can get the rack replaced. I would say that the their was a surprising power gain that I wasn't even expecting, the engine rev's up faster and feels lighter - I imagine it feels similar to having a lightened flywheel. Also, I was surprised by how easy it was to turn. At anything above 5 km/h the power steering does nothing. However, be warned that parallel parking requires a bit of muscle.
Final verdict - Even after I put in a new rack, I'm going to keep the power steering disconnected.