I believe this info can be found in Lyman's Cast Bullet loading manuel.   leadball

Shep,

  You're right in your guesses. For one thing the .25/20 WCF was first developed for the Win. 1892, since the .25/20SS was to long for that action. Also the .25/20 WCF is about .300" shorter, and about .020" in the rim diam., so neither cartridge can be used in the other.

  Coming up with a table for these two calibers is gonna be kinda hard as the .25/20SS hasn't been made commercially for many years. Looking up the data I have on those two calibers doesn't come up with anything that will match up fairly close as far as bullet wgt., powder, powder charges, and muzzle velocities go to be of any use to you to use in a ballistic program so you could figure it out for yourself.

  I've never shot Lead bullets in the WCF and never shot jacketed bullets in the SS, or vice versa, so everything wouldn't match up close enuf to be of use. If you'd like I can post some of what I have in the hopes someone can fill in the blank spaces.

  When you get up enuf data you can log onto the web site below and enter the data and do your own calculations.

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PETE

Here's some data from Cartridges of the World. A .25-20W with 9.6 grains of 2400 will push a 60 grain bullet at 2200fps with 700 ft lbs of energy. A .25-20SS with 8.0 grains of 2400 pushes the same 60 grain bullet at 1535fps, with 310 ft lbs of energy. The .25-21 Stevens, which was developed to answer the demand for a straight walled target round to those who didn't like the .25-20SS, uses 9.0 grains of 2400 for 1610fps.
As you see, the .25-20W is a bit more efficient than either of the Stevens cartridges. 
I shoot a Stevens Schuetzen Jr, in .25-21, and it's an extremely accurate round. I use either the 86 or 102 grain loading.