The full name of the warp cores you speak of are "U-Warp-SB 1" and "U-Warp-SB 2". The first one generates between 300 and 350 of warp power, the second one something around 600 I think. Both only need 100 mass.
Im playing a mod in which these power cores, or at least the first one in the latest mod version, are available. You can select them in the refit menu, and all races have them.
The only "setback" that they have is that a "U-Warp-SB 1" costs around 21000 pp in that mod, almost as much as for a stock Steamrunner!
So, you need to do lots of missions to get one, but you are rewarded with having all systems fully overpowered, at least up to and including fully armed BCH hulls.
I personally like this, cause that way the battles in SFC3 are a lot faster. For example, its fun racing around in a fully armed Sovereign with max speed of 70 and a turn rate that usually only fast Nebulas or Akiras have.
Of course, another reasons for the U-Warp Core is, we also can use the "super" weapons. (Feds have the Phaser 13S and 13F, Roms the Dizzy 5 and 5F, and so on) And we have special shields that also benefit greatly from the extra power.
Long speech, short meaning: calling the U-Warp-Cores in general a cheat is just wrong. If on GSA one uses it, but you cant have it too, its cheating. But if its available to all who play, its a very nice and legal bonus.