From what I've read, turbo plugs provide less turbulance in the combustion chamber of the cylinder. It creates a smoother flow of the vapor as the glow plug ignites. It also puts the coil further down in the combustion chamber to ensure a more consistant ignition.
There is a performance increase, but from what I understand, it's an increase that only professional drivers notice. To the rest of us, it just makes for a more expensive glow plug and we can say we use a turbo plug. (yippee?)
Anyway, a turbo plug will only work on an engine head/head button that is designed for it. It's shaped like a cone at the end that matches up to the actual hole it goes into. Like someone else said, there isn't a washer for the plug to make a seal, it depends on the beveled edge to make the seal. This type of plug requires more cautious maintenance as to not get a spec of anything on the bevel or you end up with an air leak. Unlike the normal plugs that use a soft brass washer that smashes around small particles of dirt, they still make a good seal.
Sorry for the derailing, but I thought I'd add in what I've read.