"What really surprised me is how dismal the FBI loadings were!"

This is very interesting. I posted some results obtained a few weeks ago in which I got a little better performance. My 96F was a good deal warmer than your 55F. Perhaps that accounts for some difference. Some .38 Special Chronograph Tests

I've noticed in the past that ammunition left in the sun can yield considerably higher velocities than the same ammunition left in the shade.

The Remington +P 158 grain ammunition seems a good choice for controllability and effectiveness unless a person intends to step up to Buffalo Bore's version.