All good questions that I too would like to see answers to.  As of about a week now I am shooting a 25-20 SS. I have been shooting a 25-20 WCF for many years but neither in any kind of competition. I seek loads more in line with hunting small game and pests.
One thing I can say about the two cartridges is that I believe the 25-20 WCF has been loaded to far higher pressures that what seems acceptable for the SS. That is pretty easy to see in you look at suggested loads and velocities for both cartridges using the same bullet weights.
I load the Lyman # 257420 bullet to well over 2000 fps for a Winchester model 1892 rifle in WCF but the highest velocity loads I have found with that bullet for the SS max out at about 1600 fps.
I have a Stevens model 44 that has been sleeved to the WCF but I will set a velocity limit in that rifle at about 1500 fps with the same bullet and maybe better to stay around 1300 fps.
  It depends on the launch platform on what chamber pressures should be.  Consider that I load a 2-R Lovell in a model 70 Winchester to about 3200 fps with a 50 grain jacketed bullet using the very same Jamison 25-20 SS brass. The model 70 is about a 60,000 psi launch platform where as even the strongest of the original Stevens rifles the model 44 1/2 falls well short of that limit by about 30%. I think if the 25-20 SS was chambered in something of similar strength as the Model 70 Win then it could move to a different class of cartridge somewhat similar to the 25 Copperhead in ballistic performance.

I shoot with quarter bore guys, and some are looking at switching from the wcf  to 25-35, main reason is the case is longer, so less issues with dumping loose powder in the action. I have seen the guys shooting jn the high 240's, and a couple 250's i think. I was advised to stay away from the 25-20 SS for 2 main reasons, brass availability,  and barrel twists. The original rifles in the SS were slower twists, so you have to keep the bullet weight down. The WCF is easy to get chambered in a 1:10 and shoot heavier.

If you are building from scratch, and have the brass, i don't know that there would be a detriment to building a fast twist SS

Thanks guys I'm thinking

Rick