I got a plasma cutter a few years ago and I love to just play around with it to see what it can do sometimes... but it can easily cut thru an inch of steel. So far the thickest cut I've done with it was about 1 1/4" thick but it took a while.
Cutting normal stuff like 1/4" it cuts like butter, and it can zip thru thin sheet metal so fast you can actually touch the metal right after its been cut, the plasma flame is about 1/16" wide and it's about 15000 degrees so it can cut thru sheet metal before it even has time to soak up the heat.