I'm just gonna resurrect an old thread I stuck up on this topic. Post number 42 contains results of +P handloading efforts.
Some .38 Special Chronograph Tests
I used to enjoy sampling all the powders and combinations, sitting at the reloading bench imagining the benefits as I assembled new loads, but came to the realization years ago that it was simpler and more efficient to stick with what I know will work well. 5.4 grains of Unique behind a 158 grain cast lead SWC works well to provide starch in .38 Special loads. There's not much one could reasonably ask of a .38 Special revolver that it won't handle.