Greenwood Co. Cowboy, Howdy!
Yes, the plated version is plated over brass (really bronze) as you mention. Unless you lost yer marbles and put smokeyless powder or perhaps firecracker powder in it rather than 4f, 3f, or 2f black powder, you'll never stretch the frame. You simply can't
fit enough BP in it to hurt the gun. Furthermore, most of the stretching that has happened has been primarily with the brass-framed guns that fire 44 cal. A steady diet of 36 cal. loads that are maximum powder loads
could also do some stretching, but some guns would handle it for a while. The old guns WERE made of brass, but the newer QUALITY guns, like Uberti and Pietta, use "artillery" or "ordnance" bronze, which is harder than plain brass, so the repros are generally tougher than the originals. Even the steel components are a better grade than the originals used. Some of the originals used iron (not steel) for some parts. The old Walkers had some made with iron cylinders - and gained a reputation for bursting. (Having said that, a surviving Walker is worth BIG buck$ !!!)
So, if ya want one - get one! And shoot it to yer wallet's contentment.
Then, buy a bigger gun and you'll have even MORE fun! (I'd recommend an 1860 Colt repro, but the 1858 Remingtons and the 1851 Colts are great fun, too. As are the Spiller & Burrs, and the Griswold & Gunnisons, and the 36 cal. Reb 1851s, and ... well, you know what I mean!)