I know that ls1 cars can drop over half a second with the right converter - is this true for the mighty 305 tpi as well? What have you all seen?

 

With a basically stock car with minor bolt ons, you probably won't see 1/2 second. At the same time a car with a high powered engine could easily see over 1/2 second with the appropriate stall speed vs a stock converter.

A good converter is a key ingredient in a fast car. The big thing is that the appropriate stall varies per application.

All things considered a converter is far from the last thing I'd do for performance, but not at the top of the list for a mild street car either. There are many other things to do first that will give you more bang-for-your-buck.