One big reason the Delawares, White Rock, and other white or light colored birds were the meat birds before the Cornish Cross took over was that the light birds give a prettier carcass than a dark colored bird when plucked. People never have liked those dark pen feathers.
A previous poster (Bubblebean11) mentioned that their White Rock were bigger than their other dual purpose birds. There are different strains of each of the dual purpose breeds, depending on what the breeders were after. I'd bet a lot of your money and some of mine that Bubblebean11 was lucky enough to get some of a strain that was close to the strains bred for better meat production. When the Delaware or White Rock were the main meat birds, specific strains were developed that gave better meat production. Those breeds are kept around more for egg production than meat production these days, especially from the hatcheries. If you want a strain of a dual purpose bird that is specifically bred to the original standards of meat production, you need to be talking to the breeders and talking specifically about what you want.
I assure you the commercial boys do not just take any hatchery Cornish rooster and White Rock hen to make a Cornish Cross. The strains that make the parents of the commercial Cornish Cross have been developed for decades and continue to be developed. Even if you get the best the breeders have to offer toward maintaining the original meat production Delaware or White Rock, you are not likely to get the production the commercial boys do from their specially developed lines of these breeds. So, yeah, if you are looking to get the same production the commercial boys have by buying run of the mill hatchery stock instead of investing a few decades of breeding and research, you are likely to have disappointing results. If you are expecting to get the same birds from hatchery stock that you can get from breeders that have invested a lot of time, money and research into developing specific strains of birds with "enhanced" characteristics, you are likely to be disappointed.
Some hatcheries probably sell Cornish Cross chicks that are somewhat close to what the commercial boys use, but they will cost more and probably not be the same. After all, do you expect to get a grand champion show bird from a hatchery?
With all this said, you can probably get a better meat bird from making your own Cornish Crosses, especially if you like the breast meat. You can always try and see if you like the results.