All the published loads I find for 148 gr double-ended wadcutters (DEWC) are low velocity target loads.

Can I just use a powder charge listed for a heavier bullet at higher velocity?

Alliant's site lists a 6.0 gr charge of Unique to push a 158 gr LSWC to 1034 fps.

Can just use a copper-plated DEWC instead?

The major problem with pushing wad cutters faster they are normally very soft and well lead badly. Using a copper plated bullet would help eliminate the leading.

But using a heavier bullet load and being carefull one should be able to increase the vel.

Unless they are the only thing one has there are a lot better bullets out there for self defense.

As most reloaders do, I tried to find the "perfect" self defense load long ago. I tried wad cutters both double ended and hollow base. I even inverted a hollow base wad cutter and put a gas check on it. My testing was done in a "Fackler" box using gallon size freezer bags and normal saline. I found out that a Speer 140 grain JHP was a good compromise between penetration and expansion. It has enough velocity to keep the trajectory fairly flat within usable ranges, fast enough expansion to be lethal in a body regardless of which shot is taken into the vitals, and loses enough of its energy to pose acceptable risk of ricochet after impact. It does carry enough energy to penetrate two people so the accuracy that I get from it is, as always, necessary.

You can drive a lead bullet fast enough to be a good self defense round but they don't expand well unless you limit penetration (inverted HBWC).

After doing a lot of testing also, I settled on 125grjhp with bullets in the 140grjhp range as a close 2nd if I wanted more penetration for the 357.

Don't waste your time IMO. I did the same type of (very extensive) experimentation years ago and found the DEWC to be unreliable expanders, leaded the bore of my 357 at much above 800 fps and could not compete with a jacketed bullet even then. Bullet design and construction is much better these days and very reliable. Buy some Hornady Critical Defense or Critical duty ammo and save your time by practicing at the range.

Good luck and safe shooting,

Paul