Magnum ammo will be significantly more powerful than .38 Special even out of a short barrel. I did a lot of reading before selecting the barrel length for my SP101, and depending on who was telling it and the ammo used, the .357 lost 70-100 FPS between a 2.25" and 3.06" barrel.
In short, a .357 will have significantly more oomph than a .38 out of the same length barrel. This holds true even if there is no barrel and you were to fire the bullets side-by-side out of a chamber alone.
But more barrel length will help the velocity of either a great deal.
Offered for what it's worth from a master armchair ballistics researcher.