I'll be the first one to admit that using a Ransom Rest is a bit of an art, especially with semi-autos. It has to be set up right, and settling shots have to be fired to seat the gun in the grip inserts, and there are a few other tricks as well.
But, if you want to know if load B gives a 1/4" better group than load C, there is no better tool.
Oh, and by the way, although slide to frame fit acconts for only 10-15% of a guns accuracy, I'd get that fixed before you bother with Ransom Rest testing.