Also, Dan The Fastman has some TC's for the 5.2/5.9L's (44/46RE). I haven't heard of much performance transmission components for the 45RFE trans except for the shift kit. Though I should think that there would be a few TC's since that is the essentially the same trans that's behind the 5.7.
Torque converter connects the engine to the transmission. It is a fluid coupling. The way it works is as follows: Imagine two fans one next to the other. If you turn the first fan on and have it blow right at the second fan, the second fan blade will start to rotate. This is exactly how the torque converter works, but instead of air, the torque converter fan blades are "blowing" hydraulic fluid (transmission fluid to be precise). The advantage of this setup is there is not a hard mechanical connection between the engine and tranny. This fluid coupling allows damping of shock loads preventing damage to the engine.
They don't generally add HP, they allow you to launch better. They become stall converters. This allows you to reach a higher RPM before power is applied to the wheels, which means you get to your peak power band faster and can launch harder.